We are going to DL in Feb, with our 4 and 6(three weeks shy of 7) year olds. We normally go to WDW every year and have always taken a car seat. On our last trip in August, we did not rent a car and we were doing the Land/Sea cruise package. It was the first time we did not bring car seats for our kids. The kids were fine on the plane. The car service that we used at the airport provided car seats. If you really can't see going with out them, get the Graco booster seats and just bring the base. They larger seats actually sit very high up, and interferes with the tray coming down. This makes eating and coloring complicated.
I always pack each kid a back pack. In there I have coloring books, crayons, snacks, drinks, and hand held games. I try to pick up little things before, like new sparkle crayons or something to surprise them. Trying to keep them happy is the way to go. If they get bored, then they get antsy. Then you can't relax. Last time we were on Jet Blue and the kids wanted me to buy them cookies. It was a stupid amount of money, but they were so happy and they kepy quiet after that.
In DL, don't go commando. Let the day flow the way it flows. Oncw we tried to get the little one to nap, and that was a nightmare. She was better going until early evenings, have dinner and then off too bed. Once we figured this out, it was much easier. Too many parents don't really listen to what the kid is trying to say and the kid gets over stimulated and tired.
Good Luck!