Goldilocks, I can so understand how you feel. I've been there although your circumstances are even more emotionally loaded. Of course you are totally right on principle BUT... your reaction is only giving them ammunition to justify that you are the problem as you react to their provocation. Don't rise to it however much your emotions are dictated your response.
Don't use foul language in response. If you get to that stage that that's all you feel you can do to get back to him, just don't respond at all. If he persists, still ignore. In the end, by ignoring him, you are in control.
Also, don't let your frustrations and upset show to her. By rolling your eyes to something she said that might have been kind words, you are showing to be the difficult one. Instead, it would have been better to smile, give a quick meaningless response and leave it that.
I know it is hard, I really really know, but I can assure you that it works. I've been there and come out on the other side. I can now look back and feel pride that however much my ex provoked me by being unreasonable, I manage to keep a dignified position and although my kids wouldn't say anything, I know they are now old enough to realise it.
It does get much easier with time, and I have to be honest, it was especially so once I got into a relationship myself and therefore didn't feel so alone in it although my DP never got involved at all (and I wouldn't have wanted him to be).
So tonight, go there with your head high. Don't respond to any of his texts, you don't have to. If she speaks to you, stick a smile on your face so that all she can say (if not to other but to herself) is that you are actually a nice person. Don't forget that she could be the next one betrayed by your Ex, and you wouldn't be the first one to actually become close to the woman who contributed to wrecking your life.
Being a single mum on your own is very hard, so make sure you also do things that bring you some happiness as this makes it easier to balance the * that comes with it too.