Im sorry this happend to your baby
I dont know how old the child is, if he is just young its understandable but if he is older then your right he should know better.
And so should his mother if this is her business, they will have dogs around the house alot and she should teach her kids how to treat a dog.
But lets calm down.. its only hair and it will grow back.
Accidents happen and we learn from them, just dont leave your pup with this person again.
I agree that she should have called to tell you and ask what you would like her to do first though.