I started my two 14″ One On Inbreds as soon as they were off 20″ Isla’s
I bought used 100/120mm SIDs cos they are good and light and dual air allows you to drop the travel especially when the kids are light.
I bought some used 24″ wheels (now for sale) that were also decently light and made them tubeless
I fitted 1 x 10 Deore clutch mech/zee shifter, XT 11-36 cassette, deore brakes with a 30t front ring. (Later fitting a double for faster bridleway/road stuff)
I got 140mm Cranks from Bikesmith in America for £80 knowing they were good until the kids are at least 5’5 or 5’6 (square taper but the cranks are light and sware taper isn’t a bad thing)
SDG I beam because massive adjustment and light
35mm stem from Superstar and cut down One One flat bars to fit width.
The bikes came in as light as a 24″ Isla with all of this decent kit on and I had kids on good geometry bikes, with forks that worked and disc brakes that work with one finger in any conditions. All up cost me about £500. Even managed to colour coordinate it to make them feel good…
I did it in the knowledge that with the addition of 26″ wheels and a wider bar we would have years of use and a really good bike. quality 26″wheels are now as cheap as chips and I had some brilliant Hope Hoops on Stans Alpines, CX rays lying around as I couldn’t bare to part with them for what they were worth…
So as they grew, I added the 26″ wheels, then I increased the fork travel to suit…no more new bike all the time and decent running gear…
Then as they became more confident and 26″ quality frames etc became cheaper still…I transferred all that running gear to xtra small FS frames whcih is what they are on now and very happy.
I still don’t think anyone does a proper spec and geometry kids bike for me at the money you can do it for yourself if you select really really good, but not necessarily new, key parts like forks and wheels and even frame.