Yes - I was a bit worried about cake-eating. But where he is living he can't put on his own children's party for Youngest, and Youngest wanted him there, and to be honest it suited me to have an extra pair of hands on the day. I expected no more than civility and he was able to provide that - and more - and that was plenty. I also didn't wait on him or flap around him as I had been doing - my focus was on the kids and his was too, and that was fine.
I am not sure how I have decided I will handle other events with the children going forward. I need to balance what is best for them with what H can provide and what kind of interactions I want to have with him. But this felt positive and like a good line in the sand had been drawn.