That's kind of a tough one. I would be inclined to go with your idea of waiting. But I can see his point also. Have you actually filed papers?

If the kids were littler, I would definitely wait. If they were over 13, I would tell them sooner. Where they are in age, it's less clear. If you tell them now, they might feel they have to try and talk you out of going ahead with this. They might feel very sorry for their father and dread his leaving. You and he might be provoked into arguing by the kids' reactions. The kids don't need to be placed in a position of feeling they have to choose sides. I think a lot depends on how well he and you can keep your cool over the next few weeks.