Squidrific,
Unfortunately if you can't change your schedule (although you could still change resorts and just go and hope she doesn't find you I suppose), this may force you to just bite the bullet and confront her. Just very plainly tell her, "I'm sorry but you are not invited along on our trip to Disney. This is just for me and my DH to bond with the kids." If she mentions something about having already purchased tickets and airfare, just say very bluntly, "well you did not tell us you were planning on going along before purchasing those, so I'm sorry if you will be out money, but we can't invite you this time". And then if possible, get away as quickly as possible, trying to avoid an extended argument, lol. After this, she will then have to decide if she wants to confront your DH about this and try to overrule you. If she does, then it will be back on your DH's lap to deal with. But hopefully if you've told him what you've done in the meantime, he will choose your side over hers, and then it will be over. If he chooses her side, then there's more serious problems........more than can be dealt with here.....lol. Anyway, that is what I would do, and see how it plays out.
Oops, edited to add- sorry I forgot you said you were going without DH, but I still think everything here in the suggestion applies. She still could try to confront him.