Many headset manufacturers make child headsets. I have a couple of Softcomm
child headsets. They are useful not only for children, but also adults that
have very small heads (you know who you are).

Car seats work very well in most small planes.

When you have small children on board, make your descents very slowly. The
planes are unpressurized. I know of one guy who ruptured both of his kids'
eardrums by hot dogging the descent.

Small children have short attention spans, so don't make the flight too
long. If you are taking them on a long trip, make sure they have something
to do.

Kids like airplanes because they have not yet watched enough Hollywood junk
to make them afraid. They think it is cool that every seat is a window seat.