Glad you will look after the kids. I find that kids tend to not understand adult problems. They are children and may connect the dots wrong. They might blame themselves like they are the reason the family feels "off." And not even getting their own room to take a break/time out because the mom is in there all depressed all the time... That's not really fair on the kids.
It's not like they can move out away from the wacky on their own. And let y'all solve your things yourselves. The kids are kinda trapped there.
Did they SEE that bf abusing wife's GF when wife went over there with the kids? They may have seen/heard stuff over there that they don't understand or frightened them. Have you talked to them about it?
Galagirl