If you worry about HIV merely because of a bite, you worry too much. Is it possible? Yes, absolutely. Can you pop a question to the parents, of course. Freedom of speech. Do they have to answer you? No. Freedom of not to speak.
Unless you can compel an answer, forget about asking. What if your child one day bites someone else? scratch someone's arm? Do you want people coming to you and ask: Do you have HIV? STD? Did you teach your child to bite? ...?
Biting happens in every, every datecare. It is part of life, part of children growing up. You worry not too much, but way too much.