Starting them young ?

both my boys have been on our bikes since they were tall enough to touch the pegs. my eldest just got his first 50cc yesterday, we had our first father son ride out today, I didn't get out of 2nd gear, but all good.

little 50cc dirt bikes are the trick, you can even get them with kill cords, so you walk alongside with the cord in one hand attached to a breaker switch on the bike, if they speed off or loose control you just pull the cord and the bike cuts out. really simple.
other ideas is to get them in a Motocross club.

in France you can legally ride a 50cc on the roads after a 4 day course.