In your position, I'd get a report thrown together ASAP, give it to the parents of the child who was bitten, and apologize for not having it immediately. Let them know that you'll be ensuring any further incidents are recorded and that, of course, you'll notify them immediately if their child ever may be in need of medical care. (Was the skin broken? If so, there's a risk of infection and the parents should have been called right away. Otherwise, just a report at pick up is fine.) Be calm and professional when you talk to them--don't over-apologize or make it sound like you're not sure how to handle this.
If they're on the warpath about the other kid, stand your ground. Don't let them know who did the biting. If the kid who bit has been doing this often, you'll have to decide how to handle that--the parents of the bitten kid should have no role in this.
I've been recommending
this book to everybody; it helped me a lot. It addresses the factors that lead to persistent biting, potential solutions, and how to deal with communication with parents. In that way, it's useful for all kinds of problem behaviors.
The parents of the child who bit and the child who was bitten should both be notified, but don't ever tell who bit. Don't downplay how severe the bite is, but stay calm about biting--it's inevitable, and the worst thing about it is how outraged/panicked parents get. Don't label one child a "biter." Instead, identify possible reasons the child is biting and work to mitigate.
Google "day care incident report" and see what other care centers use. Train your staff to record this stuff! My daughter's school has a simple form that says:
Minor incident report
Date of incident
Time of incident
Adult who was present
Description of incident
Treatment provided
She's brave AND clumsy, so she comes home with one of those every day.
