Did the mom witness the bite or is she going by what her son said? The reason I ask is because the dog may have just been going for the food and accidently got the boy. I know my lab was the most gentle thing in the world but when food was around, she was a real chow hound and would go for it without thinking. She accidently got my fingers a couple times, but it was nothing aggressive.

Do you have contracts you have people sign when they buy a puppy? If not, it might be something to consider. That way you could have a clause saying you had the right to take the puppy back if you felt the home was not working out.