My granddaughter started out on a VOOM 4 last spring.
I switched the 8 speed SRAM grip shifter for a push button shifter and she's really getting the hang of how and more importantly, when to shift. I've been raising the seat incrementally as the has grown over the last year (mostly in her legs) and she's about to outgrow the 20" frame. I just placed an order for a 24" Giant XtC Sl Jr.
3x8-speed
Hydraulic disc brakes
Air fork with progressive lock-out
She should be able to grow with this for a few more years.