Glennia's hack should make summer flights go a little smoother:
If you are traveling on a plane with a toddler or preschooler who likes to kick the seat in front of him or her, it might be because their legs go numb from dangling from the carseat for a long period. Put a small carry-on bag under the seat in front of the child, and after the plane takes off, pull out the bag to use as a foot rest for the child. If they insist on kicking, direct their kicks down onto the bag. The person in front of you (and their kidneys) will thank you.
Also, remember to let the child walk a bit during the flight to keep the circulation moving in their legs.