Mine (now aged 5 & 3) were introduced to flying at about 6 months. Don't forget to take a dummy. Sucking on it helps to clear their ears.
A couple of years ago I had cause to do a few night circuits in a C402 and took the family. The most exciting part for them was the fact that they could see McDonalds.
They would also collect me from work and ride up front when I took the aircraft round to the hangars. They were convinced that they were steering by turning the yoke. Once I let them kneel on the left seat while I slid down in the right (onviously I could still see over the panel). Should have seen the engineer's face when we taxied up to the hangar!
[This message has been edited by Flintstone (edited 04 September 2000).]