If you can find it, a "boonie bike" would probably serve you better. It uses a standard 4-cycle engine like that on a lawnmower, so parts/maintenance is easier.
That having been said, since it sounds like the boys will be riding it more than you, don't bother with a 4x4. Get them an older Honda 250 Fourtrax. It uses a chain drive, has a relatively small engine and the "look" I'm sure the boys want, along with decent fun factor. The big 4x4/utility machines are expensive and complex, and not well-suited to the ministrations of teenage boys.
Sorry, I should have said TRX250, 250EX, or similar. IOW, a two-wheel drive sport ATV. You don't want the utility ATVS, as I said before, because of the added complexity of the shaft drives, transfer cases etc. They aren't cheap to buy, and probably won't be cheap to maintain/repair. The two-wheel ATV will be at least as much fun, will withstand teenager punishment better, and will probably get more use in the long run.