Ratow -
First of all - you are going to have such a great time!!!!!!! I haven't travelled with so many kiddies, but have with 3 enough times to have some tips. I just got back from a short trip with my best friend, and her two little ones (1 & 3, girls). Believe me - having two strollers is a LOT to deal with, but you have to have them. Even your 4 year old will want to ride some of the time - so if you rent, you may want to rent one of the double strollers - it will be well worth the money.
We find that we actually don't head back to the rooms midday -- the kiddies are perfectly happy napping in the stroller, and we spend lots of time in air conditioning so they don't get too overheated. She has two light-weight, full function strollers - that way the kiddies can lay down for their naps, have a great shade and we have a big storage bin for all the stuff you have to haul with them. Also, my girlfriend found these AWESOME clip-on fans (I think at RightStart) for the strollers. They were something like $16 a piece and worth EVERY penny. They really moved the air, AND the kiddies loved to hold them. Also, ya gotta go to Walmart and get misty fans. The kiddies will love to play with them, and it's a ton cheaper than getting them at the park.
Listen to the folks who told you to rent a car (if you haven't driven one down - I can't remember if you said you had a trailor or RV). The bus transport is great for adults and older kiddies, but really tough on the little ones. Plus, it adds a lot of time to your day. From the campground you are a MINIMUM of two bus connections to get anywhere. I stayed there in November of '00 and transportation from FW can be tricky - I was glad I had some recent experience, because we all would have been lost! Check on
www.mousesavers.com for good discounts on rental cars - especially on mini-vans.
Use the monorail when you can. Also, you can monorail it to Epcot, walk through to the International Gateway to the Boardwalk, and catch a boat to MGM. You can also walk from the Boardwalk to MGM - it's a pretty hike, but a fairly long one. On the monorail and boats, you don't have to fold up strollers, saving a lot of hassle.
Take lots of snacks into the parks - it helps avoid that meltdown. The food thing is a HUGE, as one poster mentioned. We've found that breakfast is especially hard, as not many places serve it, so definitely plan on pop-tarts or something before leaving (unless you have a PS, of course!). Since you have the trailer, you should be set for simple food and snacks - it will save a TON of $$$.
I'll agree again with everyone -- schedule a few things (PS's, shows, Illuminations), but be flexible on everything else. You are gonna have some big fun! If you like Margaritas, I highly recommend getting a couple from the stand in Mexico at Epcot (lime is my favorite) and find a great spot to watch Illuminations (covered in bug spray). That is the life!!!!!!