In the past I've tracked him down to see if he wants to see them.
Thinking that's not such a smart idea.
Nope, let him make the effort and the arrangements. If he doesn't call and make arrangements within a reasonable amount of time, go ahead with your business and don't bend over backwards to accommodate his whims. Like others have said, he doesn't appear to be interested in managing this relationship on his own, and he's endangering the kids by watching them while intoxicated.