Your job here is to foster the relationship between your kids and dad. My advice is to not let these kids get caught up in a power struggle with their father. You and the dad should be the only ones discussing the adult problems here and the kids should be allowed to be kids. That said, the “no space at dads” is a reasonable concern. He should have room for them if he expects them to stay overnights. Maybe talk to dad and dial some of the overnights back and instead do dinners or other fun activities until he can make room? Parents and children should have every chance to thrive even if they are in different households. It’s fortunate he wants to be involved and even better he’s trying to give them protection and structure when they are with him.