I have done this since we split 2 years ago, when ds was 2 months, really because of geography - we're up north and he's in London, so 'commutes' to see ds every 2 weeks.
At first, he was incapable of more than a pram push on his own, so I had to be around. Then once ds began walking, he could manage the park and so it went till he broke his leg last October so was physically incapable of being alone with a very active 2-yr-old!
And then, he announced he was getting his solicitor involved - cue horrific row.
He has never been one to take advice, and can't see that when I suggest he do something a particular way, it's because I do it everyday and have found a good way to do it, but he just takes it like I'm telling him off.
He does stupid things without thinking about the repercussions, like getting ds to kick at his face when on the swing, and of course daddy pretends to have been kicked. Of course ds thinks it's hilarious, but not exactly a good exapmle to set...
It's really hard Norks and I would really just like to take a hit out on him, but I think as you say, you have to encourage him to take the lead more, and organise time out for him and them, and for all of you together (I know...horrific!) and then you can show him how you like things to be done.
Try and focus on the fact that they (presumably) like being with him, and he them, so as long as they're safe, he'll just have to be left to get on with it.
Hark at me giving advice! Struggling so badly myself.
Good luck!