That kind of depends on how old the kids are and if he's bigger than their father. Even if they were being terrible, someone guy puts a hand on my kids and I'm going to hurt that guy. crashwg could be a small guy, or the kids could be old enough to defend themselves.
He should just tell them the truth, that he's sick of them around so much. It's the hardest upfront but the least damaging in the long run.