Ages 5–7: weight is everything. Look for something under about 2 lbs with a shaft that collapses genuinely short. Expect shallow finds and short sessions, and go with them. See our 5, 6, and 7 year old guide.
Ages 8–10: the sweet spot. Kids this age have the patience for a target ID screen and the arm strength for a youth-sized machine. This is where a detector genuinely becomes a hobby rather than a novelty. See the 8, 9, and 10 year old guide.
Ages 11–13: stop buying kids' gear. A real entry-level machine with the shaft collapsed fits fine and won't be outgrown. See the 11, 12, and 13 year old guide.
Teenagers: buy the adult detector you'd buy yourself. Anything branded for children will be resented, and rightly so. See the teenager guide.