With our little guy we went to Disney when he was almost 8 months and did the lap child thing, not bad, he was a little smaller, and slept more. We went lap child again in April when he was almost 13 months. This time he was quite a bit bigger (he is 27#12oz now, and 33" tall). Well, he is a real calm little guy, and it was still hard. There is very little room with a not small size adult and a child this size to move around. I felt like everytime he moved he was bumping the seat in front of us, and bugging the other passenger. He slept a bit both ways, but it was hard to keep him in mostly one position as not to bump the seat. My husband and I shared the job, and I still prefered not to do it again. For our trip in August we purchased a seat for him (Southwest out of Phila $59 each way). He is used to his car seat and hopefully it will be easier this way. Good luck.
Donna