Less then 2 weeks and we take a 7yr old 4yr old and 2mth old!
We think we are crazy but know we will have fun no matter what happens!
Take full advantage of the parent swap passes, not only do u get to go on in the fast pass line, u can take up to 3 others with u, so your 3.5yr old can go on with you as well.
Plus some of the other rides like rockin roller coaster and such all have single rider lines u pretty much zip through, so u can switch off and the lines are usually not very long. one can take the kiddies and one ride.
I don;t see why u would need to spend a lot of time in the care centers, I will be nursing my littlest, and don't plan on using them unless they are real convient, maybe just for diaper changes.
From everyone elses posts I have seen wraps or slings are helpful on the rides, I had one from when we went to DL when ds2 was 18 mths, and it worked great there when he just wanted to be held at the end of the day and was tired of the stroller.
Its one vacation that is designated to kids of all ages, so sit back and enjoy!