I agree with mchames, pack the benadryl!!! I have a 2 and 4 year old and they slept from shortly after takeoff until landing. The one time I didn't give it to my daughter a few years ago she cried the entire time until the person behind us gave her a box of animal crackers, then we landed (I had packed all kinds of snacks for her but the animal crackers did the trick). I made sure I picked up a box for the return flight and gave her benadryl.
As for what to pack I bought each kid a plastic colored file folder at Wal Mart (the kind that hold bills, etc) and put small coloring books, stickers, crayons, and printouts from PlayHouse Disney (Bear in the Big Blue House, Rollie Pollie Ollie, PB and J otter). I also packed Happy Meal toys this way if we lost them it was no big deal. Pez makes Mickey holders and now Pooh and friends which I packed along with Mickey lollipops (Kmart/WalMart), lacing cards, books and a wipe off board (crayola makes a few sizes, some are plain, some have games printed on them). A favorite doll or stuffed animal helps too. A new spillproof cup/bottle is exciting and helps with takeoff and landings. Gerber makes spillproof sports bottles which are good for the older kids too, and then you can take them to the parks. In their backpacks I put a supply of snacks in a party favor bag which included cereal bars, fruit chews, miniture candy (halloween was a few days before we left), Pez dispensers, lollipop, raisins, crackers. The idea was to let them pick from the bag whatever they wanted and I knew there was a mix of good stuff and treats. All of this planning and like I said, they slept the entire way. It was worth it, and I can only hope this January it will go as smoothly.