Have taken my 13 and 11 year old neice and nephew flying. For me the key was making sure they got the maximum benefit from the experience. Explain as much as possible in undertsandable terms, involve them as much as is safe such as explain the walk around, what is safe and what isn't. Actually go through the whole pre-flight brief, e.g. fire extinguisher, first aid, opening the door.
I had massive smiles all round after the flight that included a land away (and a go around - not my fault) so they could swap over RHS.
In essence I agree with skydriller.
The more we take up, the more chance we have of encouraging the next generation.