Solution:
Brand new Brodie Derelict ( short frame) – jump bike ( seat height perfect at its lowest saddle position and enough room for the bike to grow with the boy through into teenage years…. even small adults are comfortable on this bike)
Bonus: Came with High grade Marzocchi Bomber Fork for a ridiculously low price, as old stock and paid only around 30% of RRP. 
Con: Old stock sitting in a box at warehouse for several years. Back shock seal was broken… no warranty and needed to be replaced. The Juicy Brakes were also not functioning properly and – moved to Magura MT5’s for a more solid performance.
Work done – parts
Bafang HD 1000W
48v 20AH lithium battery
Custom made aluminium Battery Case
Grin Twist Throttle
Magura MT5’s.
Monarch R Mountain Bike Rear Shock 200x51mm
We note that E-bikes and especially reliable Bafang conversions are great for kids -now able to ride with parents and keep up.
for example A 40Kg Child or teenager on a 1000w HD ebike is equivalent to a much higher Watt bike for an adult.
The key here is the kid needs to already know how to ride a bike competently, or bonus if they come from a moto background. Hey don’t let your kids loose on a 1000W HD Bafang if they cant handle the Xtreme Power…. 