I'm back, on my own.

[BeingMe]

IMHO, I would say "sorry no, we have plans." Don't include him. Would he include you? Your kids will have to adjust eventually, now is a good time for them to do so. It would be confusing, I think, for them to see mommy and daddy together, and then other times not. He has made this bed, and needs to lay in it. No doing favours, no being kind, or feeling sorry for him. He left those children, now they pay the cost, and they need to see you guys separately, so there is no confusion. Maybe, after a year or so, you can do these kinds of favours, and you can also ask to have them on his days, but this is too early.

Go have a fun weekend with your kids.