I'm going in April with DS (almost 4), leaving DD (almost 1) with DH. Here are my thoughts.. He knew what it was like to be mommy's one and only and now he has to share that time... and with a difficult pregnancy and me breastfeeding, there have been so many things we have cut. I feel like this is a time to spend some one on one time with him.
Would my 1 yr old enjoy it? You bet. I'm sure she'd smile and have fun. She'd also have to nap alot, and probably want to do it in the room or be Ms. Cranky Pants. That said, would my son like to have her there? YES! But I think he can't wrap his head around one on one time either. I was feeling a little funny about it, but then I decided it is a treat for him - his birthday this year. Maybe I will do the same for DD first trip later on. Then do a family trip later on.
As you can guess, my vote is to leave the baby with the grandparents. Giving some special time to the older, lesser of the time demanding children at this point has got to be special to them - with or without Mickey!