We left our (then) 4yr olds car seat at home (Britax Regent, not airline approved) because we weren't renting a car while at WDW. We used DME and disney transportation. It worked out fine except for the fact that our connecting flight home was canceled after we had checked in with the airline. We had to take a shuttle van to a hotel the airport put us up in overnight. They didn't even have booster seats so my little guy had to ride in the seatbelt alone, which isn't safe. I was really wishing I (at least) had a booster for him at that point! It's one of those unpredictable things so it may factor into your decision about bringing something along. Even a backless booster would be better than nothing in an emergency. I am guessing you wanted to bring a car seat for the plane for your younger dc? The DME driver will stash it under the bus in the cargo hold for the ride to the hotel.
hth!