I'm almost positive that Disney does not allow car seats on the buses because there are no seat belts. The buses are designed to protect riders in a different fashion (by size and construction) assuming those riders are sitting in the correct way (forward facing specifically). You could keep the child in your lap, but I get that a 1+ hour ride is a long time to hold a kid down.
I'm travelling with a 6, 3 and 21M old myself in a week, and would welcome other suggestions. I have a CARES harness for the plane ride, but doubt that it would work in a bus without a seatbelt (and not even sure it would be a good idea).