I think asking H to help with the kids is reasonable on several levels. You understand enough how to back off if its not working. I would suggest starting with the more fun, shorter activities they both like to begin with. Times when H is likely to be fresh and more relaxed. Keep activites short so that if its not a good day it has less time to get worse.
He is just not looking at the kids right now. He will again further along the track at some stage. If it just not working now I do think its better to try a little later, you gave it your best shot.
It's the best you can do between helping your kids and letting things take their course.
