Unfortunately, I tend to agree w/newmama. But you know H better than us- is this a tone he's likely to respond positively to (or at least won't make anything worse)?

If not, I'd make it blander but pointed, maybe something like

I want to make sure D's party is really special and that whatever's between you and me doesn't interfere- I know you'd agree with me that she deserves that. I know it was awkward the other day at the picnic - can we make an agreement that we get along and project a positive attitude for the kids' sake at these special events?

When the men on the chessboard
Get up and tell you where to go;
And you've just had some kind of mushroom
And your mind is moving slow;
Go ask Alice...
I think she'll know.