If you must get an emtb for them, go as light as possible and give them access to a decent regular bike. Personally, I wouldn't bother - just invest in good regular bikes. Perhaps hire before buying?
I have 3 kids , currently 13,15, and 17 yo , and we've been mtb riding together at various levels since each was 9 yo. They all had a chance to ride my 2018 giant full e pro (23 kg, yamaha pwx, 140 mm travel) - NONE of them rode it for more than a year. Essentially, once the novelty of blasting up hills wore off they became frustrated with the weight - especially as their skills developed.
The 13 yo lost interest when he was 12 yo - he became comfortable jumping so he wanted a bike light enough to be fun in the air. He won't even bother with my levo sl because at 18 kg it's less fun than his 15 kg norco optic !
The 15 yo stopped riding the emtb at 11 yo - she's got my old 12 kg giant trance , half the weight so dad can wait for her.
The 17 yo has never liked the giant fugly..This was her at 13 yo on that norco optic, she had already decided the emtb was too sluggish down hill.
When she was 14 she borrowed a focus jam and we did a 60 km alpine mtb ride with 100 other emtb riders. That bike was one of the first attempts by the mainstream manufacturers to build a light emtb - 300 wh battery with range extender , and a reasonably light build (19 kg) . She enjoyed the ride, but was more than happy to get back onto her regular bike
Now, she loves my levo sl - it's almost identical geometry to her current bike ( scott genius 940) , and it's only a kg heavier. Interestingly it's not the assistance she likes - it's the better handling from the carbon frame / my upgraded suspension. She still prefers her regular scott on any ride shorter than 3 hours.
