This is a challenging conversation to be sure, but hopefully it's not the first time you've talked about appropriateness of things with the child - by 8, you should be expecting her to regularly encounter material other parents may consider appropriate but you do not (regardless of how permissive you are).
The core element of the conversation needs to be helping her understand why and how content might be inappropriate. When we have this discussion with our children (around the same age as yours), we talk about a few concepts.
- Content might be too scary. We explain how games can be scary, and can be scary in ways that they will enjoy when they are older, but aren't old enough to appreciate now. This also includes violence - we don't want them to play violent games, because they aren't able yet to fully differentiate fantasy from reality as compared to an adult.
- Content they might not understand. This would include sexual content, for example. We explain that some games include topics they don't understand yet, because they're not physically ready to understand it yet. Another good example of this is a book with a romantic subplot - even without anything sexual, most 8 year olds simply wouldn't get why the characters acted how they did, and wouldn't be interested in it.
- Content we don't feel is safe for them. The best example of this is a game with a chat feature. We talk to them about how it is dangerous to talk to people online who you don't know, just how it is dangerous to talk to people in person you don't know. Chatting can be harmless, and usually is, but there can be dangerous people who will take advantage of them. We go into some detail here; we want to be the parents of the children who identify and point out the predator, and so we tell them in enough detail that they understand how a predator might act.
The focus here is telling them why in as much detail as possible, so that they not only understand why we are saying "not right now" but also in making sure they are equipped to make that decision for themselves as to whether this is a safe thing or not, when we're not there to oversee (at a friend's house, for example). Children can understand surprisingly difficult concepts, and if you have the conversation in a positive, productive tone, you would be surprised at how they will understand you.
The other thing we do is always play games with them before they can play by themselves - which serves two purposes. One, it makes sure we know if a game is a problem or not; and two, it's a positive experience for them (who, at 8 or so, still are young enough to enjoy playing with Mom and Dad). That way the next time they get a new game, they know what to do - ask us to play it with them!
