Happens here too. My kids are 2 1/2 and nearly 4. DS1 bit DD the other day and she has a terrible bite mark on her tummy, and another smaller one on her arm. Though in his defence, DD taught him to do it, by biting him in the first place. But DD has great language skills and can talk to DS1 where his language skills aren't as good as hers are and he gets frustrated easily. I am putting DS into time out, then coming to apologise to DD. Telling him it hurts DD, isn't nice and he needs to talk to her, not bite. I give him an example of what to say and get him to repeat it to DD. I also get DD to tell DS how she feels after he's bitten her. I explain to DD why DS bit her and if she used her words he probably wouldn't have done it.