EXH has been with his GF for a couple of years. I am happily remarried and our two teenage DC stay with him every other weekend and with me the rest of the time.
His GF and my DC get on really well and have built up a lovely relationship. She seems really nice and has no other children, she's mid 40's.
I have found out via friends and then confronting EXH that he is mucking around on his GF with a woman a fair bit younger who has a young baby/toddler. He lives with GF and my DC stay there and have their own rooms.
He wants to give it a go with new woman, who still lives with her husband 
but apparently not for much longer.
EXH is not telling his GF yet and my DC still visit and are obviously unaware.
I don't really want them mixed up in this mess and also continuing to build bonds with his GF if it's all going to come crashing down. I'm tempted to say that they can't go over until he sorts his shit out. They will be hurt and upset when they find out. But in the same breath, this is not my mess or my problem to sort. I am livid for the hurt he will cause our DC, his STBXGF and I think he's thinking with his dick.
WWYD? If anything...