The website to which you refer is a Chinese counterfeit and knockoff outfit. If you receive the merchandise, chances of it actually working are pretty low.
The lowest price metal detector that's designed for kids and actually works well enough to be useful, is the Bounty Hunter Junior which is carried by some forum sponsors and is also sold at many retail locations throughout the USA. It'll usually run you from $50 to $80 depending on where and when. I designed the electronics as a major revision of what was originally (I'm told) a George Payne design, so although small and low cost, it's the real thing. They're manufactured in China under contract to our specifications, and we sometimes make them here in the USA as well if there's a rush order that can't be filled fast enough by our contract company in China. The box will say where it's made, but it's the same specification either way.
I got the Bounty Hunter Tracker IV at Hobby Lobby with their coupon they always have posted on their website. I think it's 20% off an item. Found all kinds of random nonsense at the beach the first time we took it out, with no real idea what we were doing. I think with the coupon it was $80, IIRC.