Our 3 3/4 year old (as he says) little boy is a very enthusiastic co-pilot - started flying in Dad's Bushbaby as soon as he could sit - before it was covered or had instruments or doors etc.
When he was little he used to sit on my lap in the Bushbaby with a seatbelt extension like the airlines use - adds on the existing seatbelt. Used to put doofies (sp) (little yellow jobs) in his ears.
Now he loves flying alone with Dave, but knows never to touch any "controls" (unless Dad says so). Would never think of undoing his seatbelt. Depends on the age but at almost 4, you can lay down the rules and they do understand. Now he likes interesting manoevres the most - steep turns and a few Gs are great! Once he was flying alone with Dave, the only problem proved to be that my headsets were too big for him and we added a little padding at the top so that he didn't sit with head immobile for the majority of the flight. New headsets and a bigger head have eliminated this problem. The Bushbaby also has a removable stick for nervous passengers, non-pilots and children!
Now, for seat height, we put part of his car seat (the booster seats that split in half) into the EuroStar and then the normal seatbelt on tightest setting holds him tight.
We have never forced him to go flying and Dave will only take him when it is nice calm weather.