I remember reading (it may even have been in Velovision)of a bike by a company called Renn (?)
It was a metal "likeabike" but with a small stub under the frame. When skills allowed you bolted on a fitting with bottom bracket and pedals, attached a chain and poewered the same bike.
Effectively extends the life, and introduces pedalling on a trusted machine.
Edited...
I also believe that one of the better specced children's bikes is by "Puky" who have reasonable weight frames and scaled brakes tc instead of simply fitting adult ones.