Is it counterproductive to TELL him that I want that boundary or should I just enact it without saying a word? I think the latter sounds better
You definitely want to tell him. A boundary isn't really a boundary unless it is clearly stated. Just tell him that you are not comfortable being around him given that he's engaging in an affair and that he has left you, but since he needs to be there for the kids you're willing to allow that as long as he leaves at "x" time (good idea KGuy). And tell him you would appreciate it if he respects your boundary. He will probably not respect it for long, so expect that and be prepared to deal with it.
