A few things:
1) I'm a professional photographer. On three of my trips I hauled around pretty much an entire kit (2 bodies, 4 lenses). It wasn't a lot of fun.
2) As many have said, cell phones can do a really good job. If you know how to use Photoshop, they can probably capture about everything you need, and you'll have the advantage of being able to see what you're shooting as you shoot it.
3) Photopass/Memory Maker is also a fairly good option. It's expensive (to me, at least) but it keeps you from having to carry equipment around, and it lets everyone be in the shot. The two problems I've encountered with them are a) a lack of Disney photographers when I need one, and b) when I want to get a shot that Disney doesn't imagine anyone will want...so again, no photographers around.
4) If you do want to buy and haul a camera/system around, there are two big things I would pay attention to, a) manual exposure controls (and learning how to use them properly) and b) lens interchangeability, or the lack thereof. The first requires practice to get good at. The second will determine what shots you can get annd what shots you can't. Most cameras with fixed (i.e., non-interchangeable) lens will go moderately wide to moderately long, and will capture most of what you want to capture. Test that theory before you go, however, and you can learn how to use manual exposure at the same time.
5) If you want interchangeable lens, you're looking at a system, which tends to add weight and bulk. One good option is to look at mirrorless cameras, which tend to be smaller and lighter weight. Of my many cameras, the most used is a Canon 5D Mark III; I have two friends who use Panasonic mirrorless cameras who call my Canon the boat anchor, because they can fit a body in three lenses in less space and with less weight than my Canon body alone.