I think the travel binder is a great idea (I'll have to use that in the future!). Anyway, I was stressed about this too when we went in October with a 12 month old, 2 3/4 year old, and almost 4 year old. I wasn't worried about the baby, but for the older two they each carried their own backpacks filled with an extra outfit for each and diaper for the 2 year old, and coloring books, the triangular crayons that won't roll, sticker books, travel magna doodles (I got everything from the dollar store), and some other small stuff. My daughter (the 4 year old) only played with the sticker book a little bit - she was more interested in looking out the window and talking to me about the trip (she and I and the baby sat together). My son (2) sat with my dh and ate his way from here to FL! He loved the drop down tray and really enjoyed the peanuts, cookies and juice. He was a riot to watch!
There was another child on our flight (I'd say around 3ish) and he was just all over the place! He was with his parents and grandparents (they were in front of us) and he wouldn't sit in his seat. He was up and down the aisle between the parents and grandparents and threw a fit when they tried to get him to sit when the plane was landing. They had gadgets galore packed for him (dvd player, etc) and he wasn't interested in any of it. My kids were told once the seat belt went on they were to stay in their seats! I even told my daughter there was no bathroom on the planes (4 short flights, less than 3 each) and made sure she went potty before boarding and as soon as we landed. She didn't have a problem (who wants to navigate those small bathrooms with a 4 year old, not me!). Not to start a war about only kids and multiples, but it's interesting to me how much easier it is to do stuff with more than one child...they really do entertain each other!
Hope you have a great trip!