Robx, he can really only count on himself to "act mature" in this situation. His W has shown a certain unpredictibility. I would hate to see him drawn into a situation where things go south in front of his kids. Her recent talk about his gun makes me nervous for him. I had little kids when my sitch blew up. I would never have put myself in that position.
For the record, my H and I have traditionally split the Halloween duties just as DSH described. I took the kids to church and he took them around the neighborhood. Most of the parents on our block only send one adult out with the kids. As long as he has time to fuss over the costumes and have fun with them, I see no need to force a "family time".
