How serious is the kid? If you think he will get bored and abandon this by next week, go to Radio Shack. Get the Tesoro Compadre, if he will do it for a while, and it will make a good detector for you to use as a back-up when he wants to upgrade to an even better on. Or, the Ace with a pee-wee pole. If he actually DOES find something, he might stick with it and have a new life-long hobby, so you might actually want to get a good one that works!
I'd have to agree with the others....Compadre first, then ACE 250. Compadre probably easier than trying to second guess the ACE (paralysis by analysis). Tot lots and parks. Pick whichever one you'd love to have as a backup (and be prepared to do some business with him..."I'll trade you for this electronic game.....")