OK first, we are getting a limo instead of a rental car so I'm going to let them provide boosters for 5 and 3 3/4 year olds. I'm bringing car seat for the plane and limo for the 21 month old. We'll have to store it in the room-should fit in the closet.... I've got this really great carrier for that. See here:
Pack Bac It is so much better than the car seat bags as it fits snug to your body. Last year I had a cooler and booster seat on the seat and also had a car seat bag with the other car seat which we're leaving home this trip

I know DS will be fine on the plane-he's nearly 40 pounds and now understands that he needs to sit in his seat.
For carry on I'll also have a "regular" carry on bag with my camera, diaper stuff, clothes for baby, Tshirt for me... and a backpack with books, stickers...I'll try to keep that to a minimum since the planes now have tvs on the seats. I'm hoping to put lunch size cooler in the backpack but we can carry that if needed. I may also bring my DD's(5) backpack for her stuff to make room.
For luggage I've got the nice big rolling bag. Then last year I used a duffle bag that I got free at work. It's not really meant for checking so I don't think I'll take that one this year. The cargo bag/backpack is HUGE. Its about 30-36" main bag plus 12" pockets on each end. (I tried to find a picture of something simular on ebags but ended up seeing lots more I like-MUST STOP!!!!) I think I'll take that one if DH doesn't think its too big. I could also take another weekender cloth suitcase or the small or medium size rolling bags. I think I could attach the small roller onto the large one so 1 adult could handle all the luggage. We'll also have another adult with us. She packs light-a small roller and a very small canvas carryon with paper & paper back.
I'll let my DH decide what he wants to carry to the parks. I'll have the small camera bag and water bottle holder. He can use my daypack (rather large Eddie Bauer) or we can bring our waist pack-not just a fanny pack.
I do plan on shipping a package to the resort with diapers, wipes, a few toys/surprises, and a few extra clothes. I know limo will stop for grocery which I plan to do but they won't if flight is delayed so while we can make do without water bottles and snacks I think I'll mail diapers to be safe-can't pay resort prices for those! I plan on doing laundry and try to pack light-I don't think 2 checked pieces and 2 carry ons is bad for a family of 5. We're going to
MNSSHP but I think I'd feel better putting the costumes in the luggage than risking loosing them in the mail. We stay concierge and last year the package was waiting for us in the room when we checked in. Concierge also sent a package home to me of mostly dirty clothes.
Is it too early to start packing so I can decide which bag will be best? I haven't told the kids about the trip yet but the luggage is out from the weekend trip to Maine. Maybe they won't notice.....
How do you keep from overpacking if you drive?