Hmmm, not all kids are seat kickers... also, isn't seat kicking more of a problem with lap children, which your kids are too old to be?
But couldn't both of the above problems be partly solved by strapping the kid in a car seat and strapping the car seat into the airplane seat?
Our 4yr old can do things and fly ok. Altho she isn't like 8 where she could enjoy a hand held game for 2 hrs while I sleep. Instead I have to interact w her.
Our 2.5 year old is working his muscles in that energetic body and simply needs to move. Keeping him strapped in only works for some hrs not 7.
He is 37 inches tall so his feet reach...mostly to hold them up when slouching but we gotta be on him to prevent kickage.
A car seat is an extra piece of stuff to carry and if we fly in the us we can rely on rental cars having one installed upon request. In NL family brings them on pick up.
Basically it will be easier when they get older but we love them always