I do not think you can go wrong buying your kid, or yourself, a hardtail mountain bike with good geometry. Get the best Shimano drivetrain in your price range, hydraulic disc brakes from a reputable brand, a serviceable air-sprung fork, good tires (which may require an upgrade), and a grippy pair of pedals. And then get out there and ride, ride, ride.
The first bike I went off-road riding on was a complete piece of crap. Steel rims. Rock-solid forever-brake pads. Wire-bead hard-rubber Comp 3 knock-off tires. Aspirational-at-best braking & shifting performance, and a one-thousand-pound frame two sizes too big. I fell head-over-heels, multiple times, and also in love with the activity. The hot take? Get the best bike you can afford and go into the forest.
From the most basic v-brakes and rigid fork setups to the multi-thousand-dollar full suspension grom ripper rigs with hydraulic discs, there are a few features these bicycles share, first and foremost being assembled and tuned by a professional bike mechanic. The gears will shift, the brakes will stop, and the grips will stay in place.
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