For the 5 1/2 year old, will he be more comfortable in a stroller. My 6 1/2 year old will still get one every once in awile if we're there long hours. Also if you have a disney visa (I think that was it, maybe it was DVC members) you get free stroller rental. They cost the same as the wheelchair.
You can go back and forth from park to park no problems. They do have a limited number and do run out, less often on weekdays, but there is no guarentee.
If you're staying at one of the DL hotels you can rent a wheelchair for the length of stay for 10 per day. These must stay at the DLR resort, but you can take them throughout the whole resort (dtd, parks, and hotels). No depost, but a 300 replacement cost. These also can not be reserved, but if you contact guest services and they don't have the non electric ones they will get you one from one of the parks for the length of stay.
Donna