I don't see what the problem is. There was one, with this other child and his behaviour, but now your children don't see the other child and are behaving normally again. I would maybe consider getting some sort of written agreement (court backed if needed) that these children do not see each other until the other child has had counselling or something to sort out his issues. I would ensure his issues are highlighted to some relevant authority as it isn't fun being a 10-year-old with problems.

You have your 3 children Mon-Fri (I believe, do correct me if I'm wrong), then weekends is one-on-one time with your eldest or time for you to focus just on your younger boys. Your children see each other a lot in the week, and get some extra time just with you on the weekends. How is that a bad thing if you look at it and pitch it like that?