Hi Jen!
I'm not an expert, but here's my 2 cents anyway ;-)
I usually put my 18-200 lens on the camera and walk around with that. The longer length is good if your kids are going to ride the rides alone (like Dumbo) because you can get great shots of them way up in the air
And the shorter length is good for snapping candids of your kids on the fly. So, looking at your lenses, I'd lean toward walking around with this one: 28-135mm.
Your nifty fifty might be really good for character shots, food shots, and Mickey ears shots - plus, it's lightweight to carry around
Can't wait to see what the experts recommend, though.