Like Giggler said…go with a hardtail!
The B line is a cool bike, but like so many duallies, kinda heavy, especially for a 10 year old.
My 10 year old is itching for a dually, his desktop pic is a Transition Double, changes colour every day too…But he's going to stick with his hardtail for awhile. Unless he wins the lottery.
I built up a dual 24, dual disc, steel hardtail for him this summer. Picked up a Manitou Stance Static 80mm for it, used some old parts I have kicking around, cut down some old Answer Pro tapers, had a 10 year old XTR lever/shifter combo for the back shifting/braking. I'm making him run one ring up front, no derailleur.
It was more fun finding a small hardtail frame, scrounging for some parts and building it up with him…
Ken