I've taken DNiece and DNephew on multiple trips, including Disney (now age 10 and 8). I generally have their mother sign a power of attorney over to me, so that I am authorized to make heath care decisions for them. I can send you a copy of this if you PM me with your email. I also take copies of their insurance cards (again, for heath care issues). The airlines and Disney do not require any documentation.
In terms of other ideas - my nephew can be a bit hard to manage at times. He does not take direction well, does not remember instructions, etc. To keep them occupied during travel, I had activity books, regular books, cards, personal DVD and GameBoy. This gave us enough to do while waiting at airport, on flight, etc.
I also gave clear instructions on what to do at Disney if they got lost, helped them identify CMs, and we even went up to several CM's to "practice." It was a good thing, because sure enough DNephew wanders off on Day 1 at MGM. We found him within 5 minutes (he was telling a CM he was lost) but what a terrifying 5 minutes. Also, we pinned our name, cell phone number, and resort into his clothes to keep track, so if he got scared and couldn't remember the information, at least a CM could find it and use it to contact us. My DNiece is much more mature, and knows our last name, cell phone number, etc., so I was not as concerned with her.
The only other issue we had to deal with with DNephew was lines. We were there late April, and it was not that crowded. However, he hated to stand in line, could not seem to stand still, and tried his best to drive me crazy. We worked out some word games, math games, or just anything along those lines - testing on spelling words, how many Disney characters can you name in 10 seconds or less. As long as I could keep him occupied, things were a lot easier on all of us.
The kids were 7 and 9 on that trip, so we'll see how it goes this fall with each a year older.
Have a great trip.