My wife and I really like a neighborhood day care we recently checked out. The place seems very nice and the care providers seem to do a very good job. They are organized, appear to give a lot of attention to each child, and plan activities for the children. Our only reservation is that the providers are non-native English speakers. They have a strong accent and I assume a smaller vocabulary compared to a native English speaker. So we are concerned with whether our child would get the same level of language stimulation that they would from a native English speaker caregiver. Will spending significant amount being taken care of by a non-native English speaker speaking English impact our child's language development?
My honest opinion is that if my child spends a significant amount of time in the daycare, then I would require that the staff must speak reasonably accent-free. Children learn from what they hear, and I'd not want my child to learn incorrect grammar/pronunciation/etc. because of any staff's accents.
I'd like to point out that I'm not at all against meeting people with strong accents and spending some time with them - on the contrary, it shows the breadth of the population, which is a good thing. But my reservation is that this should not be the dominant input outside the home.
I can't really imagine that a daycare in country "A" would be allowed to be operated in language "B" unless specifically entitled to, which is not how I understand your question.
But given this, I think it would only really work if the child spends a significant amount of time in the daycare, otherwise there's not enough learning opportunity for the child and it would just be "foreign" to the child, which would in turn be ineffective and also very frustrating for the child and the caregivers.
I don't see much potential of language development here. If the caregivers speak their native tongue only among themselves, the child will understand that "foreign languages" exist (which teaches a cultural lesson) but that's about it, I don't think the child would even begin to learn the language from that.
My experience is:
My parents-in-law are non-native speakers of our (and our child's) mother language and I see, that our child (which often spends time with them, on average maybe at least 6..24 hours per week) also adapts
to some extent (!). Our child is 3.5 years old and was and is (from my point of view) quite interested in speaking, language and vocabulary and started talking quite early.
So I think that it might have a negative impact concerning pronunciation and vocabulary/vocab/idioms usage of your child. That might depend on the amount of time, your child spends with those non-native-speaking people and surely also the age of your child.
That it also trains the child's understanding for different sounds and pronunciation, as some other answers assume, is possible. In our case it also often stimulates discussions about pronunciation with our child. But it is IMHO quite difficult to explain to your child that those people which it should respect (like his grandparents in my case or those people in your day care) are doing something "wrong".
On the other hand, a native speaker who doesn't care much about the child itself is not a better alternative, AFAIK.
Speaking from personal experience, I had a brief period where a babysitter spoke to me in another language when I was just learning to speak. My mother was surprised that I stopped speaking the few words that I had learned and started mumbling what was baby-gibberish to her. On a subsequent visit, the caregiver said, "oh, he just wants a glass of water." I had learned to speak enough of the babysitter's language to communicate with her. However, I quickly learned how to communicate with my mother again. It's probably fair to assume that, if that babysitter had continued living in the same area and my family continued to have her take care of me, I would have been a pretty natural bilingual. My English abilities were certainly not irreparably harmed and I certainly don't have the slightest accent, nor any hint of non-native speech patterns, that can be traced to that experience.
My understanding is that children raised in multilingual environments tend to have an easier time developing a broader range of communication skills. I have personally found that intentional study of foreign language in high school and college improved my understanding of English.
We are living in the US and I am a native speaker of English. My wife speaks Japanese to our son. I speak a mixture of English and mediocre Japanese. He's not quite old enough to speak intelligibly, but we fully expect to raise him bilingual. If I were a little more adventurous, I'd speak German with him and try to help him pick up three languages. We're fairly confident that my poor Japanese won't irreparably harm his ability to speak proper Japanese, though obviously I can't guarantee anything; I also had a few Spanish and Portuguese speaking caregivers who spoke English to me and I can say I didn't pick up a trace of Spanish speech patterns.
We have some friends in similar circumstances to us with a pretty wide variety of results. Less social parents found that their children's speech tended to develop a bit later than parents who spent more time with people outside of home, but generally, there is no "harm" from speaking two languages to young children. Some of our friends' children are a bit willful and reject speaking the non-dominant language but will generally understand what their parents are saying without any trouble. Presumably, because few, or none, of their friends speak Japanese, they refuse to use Japanese even with their parents.
A fair number of studies have demonstrated that most children learn "code switching" and are generally able to distinguish between languages. Over time, they figure out which contexts are suitable for which language. In practice, I've found that in languages I'm most comfortable with, with the appropriate audience, I can incorporate phrases or brief interludes from the language that is most comfortable for whatever thought I wish to express, so this does not surprise me.
My wife is not a native English speaker and has an accent (though lessening over time), and she is the primary care-giver during weekdays when I am at work. My son speaks like a native.
My wife's sister is a far worse English speaker and much more heavily accented (and not improving), and furthermore her husband is a non-native English speaker as well, and guess what? Her son also speaks English like a native.
Thus I don't think you have anything to worry about, because children can tell what is correct and what is not, as long as they have enough exposure to the correct thing. I realize that whether your child will get enough exposure to the correct thing is the aspect you are trying to figure out, but I think the quantity of correct exposure doesn't have to be as high as some people are saying.
I read some research lately about what kinds of language input helps children develop their language skills the best. Perhaps surprising, it was not the level of education of the parents, and not the quantity or quality of parental vocalizations. It was the amount of vocalization in response to the child's own language-like verbalizations and the lack of conflicting input (not rewarding non-language-like child vocalizations).
Generalizing, it appears that a child's language acquisition is a positive feedback loop that requires sufficient quantity of right input and generally ignores wrong input when it is not confusing. If the right inputs are provided early, then development is early. While this research point may not be directly applicable to your question, I think it sheds light on the subject, and I think, based on my own experience, that accented English is not confusing input.
My son occasionally pronounces words incorrectly, sometimes reflecting how he heard them from others (perhaps his mother). However, 90% of the time it only takes one correction by me for him to adopt the correct pronunciation.
Again, I don't think you have anything to worry about as long as you are properly interactive with your child during the time he is with you.
I am not an expert in the subject but what I have read would seem to indicate that it might have a positive effect. Assuming you are the primary care-giver, the baby will ultimately learn your language but being exposed to other languages at an early age can help exercise their comprehension skills, and advance their understanding of languages and their ability to interpret.
So, while it's not exactly the same circumstance I think it makes sense to look at the language development research around children of immigrants. What you'll find there is that the immigrants will either have very strong accents or won't be able to speak English at all while their children will be native English speakers with no foreign accent and will be native speakers of the foreign language without an accent.
I think it's clear from this that children will eventually get stuff "right" despite being around people who are doing it "wrong".
By "right" I mean the dominate forms of the language in terms of frequency (hearing native English being spoken form you, their peers, people on the street, TV, radio) and authority (you are their parents and they will accept your corrections for English over any others).
I imagine the impact is that they'll pick up a second language. I'd encourage them to speak in their native tongue.