Whenever someone asks what they should get for their little kid, the beeper fans jump in recommending adult-size detectors, which was the problem in the first place.
The only US "real" metal detector designed specifically for little kids is the Bounty Hunter Junior. Adults tend to sneer at it because it's obviously not for them-- which of course is the point of it! It's not high performance, but within its performance limitations it does its job very well.
Several years ago I was asked to find a recently lost item of kid's jewelry in a church lawn, a church that's been around a long time. It was a job for which I could have used almost anything at any price, but for a small almost-surface item in a trashy lawn, the BH Jr was a better choice than the "big guns" and the BH Jr. is what I used for the search. .....I didn't find the lost item (I think she lost it somewhere else other than where she thought): my point is that the BH Jr. is a real metal detector that I not only designed, but upon occasion have used in preference to all the other units I have available.
[Don't buy it at Big Lots, their system has it priced wrong. Widely available elsewhere in the $55-75 range, and of course support your forum sponsors.]