4 years is quite old for biting isn’t it, it’s usually a 2 year old thing? You’ve said there are “problems” in the family, it seems obvious that the 4yo is affected by what’s going on and the biting is an outward symptom. You’ve decided that you do want to say something to the mother? I guess in that sort of situation I might say something like, “Hey, that biting isn’t good is it? How are you going with it?” sort of implying that she does need to do something and perhaps leading her to ask for or admit to you that she needs help??? Then you could perhaps suggest something directly or refer her to her GP or MCHN for help? I think it’s really, really important to keep the tone light-hearted, otherwise she is likely to get very defensive and immediately clam-up?

As for your DD, if that was my child I would either cut off all contact (but you’ve said you don’t want to do this) or I’d be right next to my child protecting them preferably with my body between my child and the child that you’re worried about. I would be ready with my body to just block them out. I do stand back and let my children have lots of good old rough and tumble play with other kids, but when I know they’ve had enough and are being really hurt I step in to help and protect them.