I linked to this post from one in the Delta forum - maybe a little late in the game to comment, but I didn't see this mentioned before:

We have taken our son on many flights, and during a few of those we have put him into his carseat for safety. The problem is, his legs stick out of the car seat and are directly against the seat in front of him. No-where to move. So, we apologize in advance to the person in front, and try to minimize his movement, but kids aren't always trying to kick the seat when they bug you, sometimes they just can't help it.