We always packed small, new toys in the carry on, and we tried to make it something with good "play value". We wanted stuff that would hold the kids' interest for awhile. we did trucks or cars with moving parts, the Bob the Builder ones were great, finger puppets, things we could make up stories with, etc. It is hard, but they will sleep. We also found deserted hallways for the child to run through during the layovers. Some airports are better than others for run around stuff. There is at least one, maybe even SEATAC that has a small playspace.