Okay, multiple thoughts...
1. What you take depends upon what you like to shoot. Since you'll have family, I think swapping lens at Disney (unless you are a photographer and not a parent) isn't realistic.
2. Leave the speed light at home (unless you're talking photos back at the hotel or outside a restaurant at night). Not even for fill-flash...use your popup for that. Here's the deal...if Disney takes you for a pro (or if you're shooting commercial work or stock photography) someone will likely intervene. So tripods and speed lights are often the signal for someone to step in and say "excuse me, can I ask what these are for? Are you aware of photography policy within the park?"
3. As for me, I'd probably take the big zoom. You can attach that and leave it on.
4. Yes, some rides and locations will be dark. So you aren't going to get shots in those. But there will be a few places where you need to zoom in (b/c of a barrecade/wall) and someone where you want a shot in the distance. But mostly it's going to be a ton of people in bright light so a wide angle will be critical (unless you want to do a lot of post-production cropping).
5. Get thee a polarizing filter and do bring your lens hood. Also bring cleaning equipment (b/c it will be humid and probably hot so you'll sweat. Also bring a large ziplock bag in case you get caught out in the rain with an impromptu thunderstorm.
6. As someone else noted, these places are not noted for great artistic shots. Still....you can get a clamp that your body will screw in and then shot long exposures on rides that will produce some interesting shots...and the clamp is relatively cheap and not that big/heavy to schlep around.
7. As to the original question about padding...a towel is fine. I have a large fanny pack that holds 2 water bottles outside of the fannypack. I put some padding from a Timbuktu kit in there and it will hold a DSLR body and zoom (as long as it's not an f2.8 bazooka) plus other equipment quite well. And being in Florida, you'll want to use something like that to hold water bottles, a bag of wipes, sunscreen. Personally, I'm not big on backpacks (b/c you can't wear them in the car or on most rides--they'll have to come off--and then it's easy to forget them). But if you go with the backpack and just the one big zoom, then cushioning should be easy. A towel, a spare polo or t-shirt will do fine as long as it's in a pocket where it won't bang around or get water/juice dumped on top of it.
