most likely there will be nothing you can do. just plan on getting your nephew to replace it.
one thing though, you need to be specific as to the problem instead of saying it is not working. that covers from not being able to turn it on to broken on/off switch.
I have Garrett 150 metal detector. Last Friday my nephew put my metal detector in water and now my metal detector is not working so can any one tell me how i repair my metal detector.