Ok, I can't help but pipe in here ~~~
My experience is not with travel to Disney, but it is with traveling with small children (4 and 2).
I went on-line to book my tickets and get my seat assignments. The flights to FL had connecting flights, so four legs all together, three of which our seats were separated, close but separated. For me, this wasn't acceptable at all. So I called the airline to get our seats together. They said they couldn't help and I *had* to do it at the gate.
Once I got to the gates, I didn't have any problems with my first leg and tried to get the second leg fixed. They were able to fix it for me. However, they couldn't do the two return legs. I was told I'd have to do it at check-in.
So return check-in time comes. First return leg, no problem. Second return leg had to be done at the second leg's check-in counter. When I get there, I was told they had just assigned it to someone!!! Now remember, I tried at booking on-line, I called after booking and I had asked at each check-in section prior to this leg to get the seats changed! I was told at the last leg's check-in that they hold seats for those that don't have seat assignments when they check-in!
So to all of those who say they shouldn't have to give up their seats, no you shouldn't. However, when someone is trying and has tried desparately to get seats together the airlines should do everything possible to make it happen. Especially when they have rules that no child under the age of 12 can travel without an adult.
The seats they were holding were isle seats, too!