I still have not taken my eldest up yet (he's 2yrs 8 months) but when I do, I'll strap his car seat into the aircraft (I have done this some time ago with someone else's child and it worked very well).
I'll also make sure he has a properly fitting headset.
I'm planning to teach him to walk behind other aircraft on the ground (not too many pusher props around fortunately) and, an idea from a previous post, offer £1 for every aircraft spotted in the air!
In the meantime, he's enjoyed sitting in the plane and playing with the controls.