A follow-me tandem (which couples a kids bike to your bike) is a much better solution than giving a 9yo a poorly controlled motor.

No it's not, it teaches the child dependence on the adult instead of giving the child agency.

But a 9 year old is dependant on their parents ...?

	▲	fragmede 3 hours ago \| parent [-]
		...yes? The child bicycling their own bike, vs being attached to the parents bike is a tiny bit of independence to grant a child at that age.