The answer to this is pretty complex, as partially touched on by others. For an adult the answer would be technically no (but realistically far more complicated), because the definition of 'abuse' means that it is undesirable; but for a child any action designed to encourage sexual behavior by an adult is generally considered abuse; on the grounds that just because the child is too young to understand or articulate her feelings doesn't mean the action won't ultimately be harmful.
The distinction of abuse having to be unwanted is still sometimes relevant for a child though, and thus worth at least addressing briefly. For instance if an 8 year old was involved in some form of so called 'sexual' play with another child of nearly the same age, and without any age-inappropriate level of knowledge about sex, rather or not the play was abusive would largely depend on rather or not the child in question found it undesirable or not. Playing 'doctor' is normal and is usually not abusive for children of the same age, but if one child does not want to play doctor then it can become abusive. Some may view this play as sexual, though in reality it's more about curiosity and exploration then actual sexuality, but it can only be defined as abuse relative to the child's perception. Just because an adult doesn't approve of the play does not make the play inherently abusive or harmful to the child. Still, I presume your question is focusing on abuse from an adult not another child.
Then the answer to the question as written is yes to sort-of-yes depending on definition of 'enjoy'.
Most obviously, it's possible for a child to not enjoy a sexual activity while still defining it as enjoyable to themselves or others. Others have already touched on this aspect in some detail so I won't reitterate too much of it. Since emotions and desirability is complex, which can lead to situations where the child does not like the sexual activity, but does like other aspects associated with it, like the praise an adult gives the child for performing the sexual action, or the sense of being 'grown up', or the extra attention and play that happens before/after the undesirable sexual activity. This can result in the child viewing the overall reaction as enjoyable even while finding the sexual aspect of it undesirable.
Things get more complicated though as you consider sense of obligation and confusion about the actual activity. A child may not enjoy an action, but they may feel an obligation to the adult performing it which makes them feel they have to enjoy it. They may be afraid to tell on an adult, or afraid of the adult being forced to stop because they value the adults attention in other ways. In addition a child may try to convince themselves that they enjoy something that they actually find uncomfortable because they believe they should enjoy it, or they are disloyal or immature for not enjoying it, or because the only way to deal with something so upsetting is to convince yourself you like it as a coping mechanism.
Finally, sexual contact can lead to physical pleasure, in some cases even a full orgasm, in a child. They may therefore enjoy the physical sensations of the abuse even as emotionally or mentally the find it harmful. For any person, but especially so a child, the enjoyment of the physical sensations can confuse them into thinking that they must 'enjoy' all of the experience, or even that they can't say it was unwanted since their body enjoyed it (physical enjoyment does not make an action right or justified, but sometimes victims of sexual abuse still feel this way).
So in short, there are situations where a child may enjoy part of the situation, or simply have tried hard to convince themselves or others they enjoyed it, even as the action as a whole is undesirable and uncomfortable.
Separate from the confusion of uncomfortable actions defined as 'enjoyable' one can argue that certain forms of sexual abuse may actually be enjoyed, or at least not cause immediate discomfort, while still being harmful. That is to say there could be a situation when a child does not feel any immediate discomfort or is not otherwise significantly upset by an abusive action directed towards them. This DOES NOT mean the action was not abusive, as some abuse may cause harm at a later date even if they were not immediately harmful to the child.
For instance an adult can 'groom' a child to accept sexual action through subtle ways, such as complimenting their body in inappropriate manner or encouraging the child to ignore sexual boundaries such as nudity while playing games with them. The grooming itself may not be immediately harmful or even truly enjoyed by the child, as it still involves compliments and playing of games and the child may not find the form of game and compliment unusual at the time it occurs. However, the end goal of such sexualization is to ruin the child's ability to develop a proper self image ans understanding of their sexuality, and to make the child susceptible to later more direct forms of sexual abuse; both of which will ultimately harm the child. The harm may not be noticed until later, when the child's self image is so tied into sex and sexual image that they can not see their worth outside of sex, but at the moment the child received a compliment about their body the compliment may have been 100% welcome; the child can't realize that the pleasant compliments may harm their eventual emotional and sexual growth.
In a related concept a child who lacks any sense of sexuality may not consider an action as sexual and thus not see any reason to be uncomfortable about the action. For instance if a young child were asked to pose for a play "photo shoot" they may find this a fun game, just like dress up or making silly faces. Depending on the child the fact that they were naked for this 'game' may not register as unusual or wrong, since nudity need not be associated with sexuality in a young child's mind. Since they do not recognize the meaning behind the photos, or their being naked for the photo as being suspicious, they may not feel any immediate harm or regret from the action; in their mind it was just harmless play.
Of course this is still abuse, and not acceptable. This can still cause a sort of delayed harm, by the child being habituated to sex and sexuality which does harm similar to other forms of sexual grooming I mention above, by encouraging the perpetrator of the abuse to engage in worse actions later, or by the reaction of the child when they are older and realize the true nature of what they thought was harmless, or by the photos still being available and viewable after the child is old enough to be properly upset by someone viewing such photos sexually. The point is that in theory abuse can occur without a child having any immediate regrets about the action; and this does not mean the action should be deemed acceptable.
However, your real question doesn't seem to be about abuse or enjoyment, but rather what to do about suspected abuse, so lets address that.
First of all, you don't give much specifics about why you suspect abuse. I wish I had more detail here to better guide you. One thing I notice is you imply the child is still happy, usually an abused child will have rather obvious side effects like depression, fear, regression in age appropriate behaviors, nightmares etc etc.
In either case DO NOT ignore anything that concerns you. Far too often people ignore warning signs for fear they may be wrong or because it's too inconvenient to look into or they don't want to know the truth etc. If you have any worries you need to do something!
However, having said that, be very careful about speaking with her directly about this yourself. This is actually always good advice in all situations of suspected abuse of a child. The reason you want to be careful speaking with her is that you may be conveying messages you do not mean to when speaking to her, messages which will make her feel worse about any abuse that may have occurred
For starters if you are (understandably!) upset about the abuse, your anger at the person abusing her may be read by her as anger at her for being part of the abuse. She may feel she did something wrong to upset you and thus feel worse about being a "bad girl" who angered someone she card about. Similarly Questions about someone doing something "wrong" to her may imply to her that her actions were "wrong" and thus she was wrong for being involved in them. Saying that little girls shouldn't be doing some action may be interpreted as her being wrong for being involved in the action, rather then the other being wrong for making her etc.
If she loves the person abusing her anger or hostility at that individual can also make her feel worse because she still feels attachment and loyalty to them and does not want others to be angry at this person. She may feel bad for getting someone else in trouble. When you don't let her see the abuser again she may think it's a punishment because "she told" like she wasn't suppose to. In short, it's possible to make her feel worse about the situation if discussion is done in such a way as to focus on the "wrongness" of the abuse only; rather then focusing on her deserving choice and that it's better that she not have to do anything she is not comfortable with etc.
There is also the chance she will give you the answer she thinks you want if you speak with her with leading questions, which could lead to the wrong person being accused or misunderstanding of the actual situation.
Instead of talking with her directly I would instead seek a professional, who knows how to handle this situation better, to speak with her. A professional can both ascertain if abuse is likely to have occurred, and if so, help her to understand the abuse in context of something that shouldn't happen to her, but not something that she has done wrong. By professional there are really two your want her to see, her pediatrician and a psychologist.
Her pediatrician can inspect her for signs of physical sexual abuse, such as tearing of vagina, missing Hyman (which by it self does not prove abuse!) or contracting of STDs. Such an exam can confirm certain kinds of abuse with a high degree of accuracy, but it can not rule out abuse; as many types of abuse will not lead direct physical harm. However, the other advantage of contacting a pediatrician is that they can likely point you to a psychologist who can speak with her in detail.
Ultimately a psychologist is the best resource, ideally one that already works with children who have been sexually abused. They will know how to ask the right questions to determine rather she was abused without putting her in a situation where she feels she has done something wrong. If you believe abuse is likely I would look into contacting one.
However, until you have seen a specialist I would be careful about discussing this with her directly. I would limit my questions to things such as if there is anyone she doesn't like seeing or anything she would prefer not to do with others (without saying anything about what sort of things she may not like) and see what she says. Most kids will mention lots of things like not liking to clean their room or eat vegetables, and wanting to not be babysat by the boring nanny that doesn't have TV etc, silly things. But she may hint at wanting to avoid someone that she is uncomfortable with due to abuse. If so I would not push too hard on why or what happened, again as a non-expert you may risk use of leading questions which bias both her answers and how she views the situation; simply consider not forcing her to be alone with the individual until she sees a psychologist or doctor who can look into the odds of abuse and better help you to discuss the topic with her.
If for some reason her parents continually refuse to have her speak to a specialist then you may wish to look into more direct options, like contacting child services; since there is always a chance that one of the parents is abusing the child and actively resisting seeking help for the child. However, I would at least attempt to arrange for her to see a psychologist without child services first; while child services is usually very delicate about this I think a single professional psychologist can be more delicate in handling the situation; particularly when ascertaining if abuse has occurred at all. Still, I stress that it's better to contact child services then do nothing if no other actions are being taken and you truly suspect abuse. It's better to say something and be wrong then say nothing and allow any abuse that may be happening to continue.
