This would be great to add to my 9 year old's bike. We couldn't do long bike tours anymore because he would stop biking after 100m, telling is he is completely exhausted and must go home. I was looking for something like this!

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.

Yes, we already have this (Follow-Me - the swiss company). However, my 9yo and his bike is too heavy for that. I was able to carry/drag his bike from about 3y till about 6yo.

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 ...?

...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.