Lando 
Lando
buy an old cj2a, there should be a few of them up in your area.
I JUST bought a 2005 Arctic Cat 500 TRV. I've been searching and doing the homework for two years. It's a two seater, so they can both ride safely and comfortably. Bass Pro Shops is having a clearance on them right now if there's one near you.
That said, my brother-in-law hit a bridge abutment and had massive internal injuries. He survived, but spent about a month in ICU.
We still ride there, but believe me, we are careful.
With two boys are you going to be able to get away with just one vehicle?
Make your own choice on model or whatever, but put a GOVERNOR on the throttle to keep it under a certain level until they are fully capable of controlling it at speed.......
Where's your property?
Yamaha Blaster 200 cc. Cool and fun. It's not one of those heavy old clunky things like the Gator or big Honda 4WD's. It's small, light, sporty, and easy to operate.
Check with you local dealers to see if they can point you to the one day riding courses. I took my boys to the one day Motorcycle Safety Foundation course and it was well worth the very reasonable fee charged.
Uh, Dad, better make that two! We've got three boys pretty close in age, and I can't remember the last time I bought one of anything.
Wait until they are 16.
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.
For your consideration
http://www.google.com/search?client=safari&rls=en-us&q=atv+accidents+children&ie=UTF-8&oe=UTF-8
I have a 5 year old Polaris Sportsman 325 that is perfect for various uses (some trail riding, hunting, work around the property with a small trailer attached, plowing the driveway, etc.) My 13 year old is at the point where he can barely handle it, so I might recommend something smaller for now for you. The Polaris CV transmission is also nice so you don't have to worry about changing the gears as you are working.
Whatever you buy get two helmets that fit and a good chest protecter. Honda is advertising a nice four wheeler now that you can set the throttle back on so they cant go so fast , however anything a father can set back can be figured out by the boys.
I agree with the poster who said wait until they're 16.
They have to have something to look forward to. Or at
least let them earn it with chores. Don't hand an ATV
to them or they won't value it and then they won't be
careful with it. Been there, done that.
Count me in on the 'wait till they are 16' side. At minimum, do not let them ride out of sight of an adult. Kids will be kids. And testing their limits on these machines can lead to tragic consequences.
I work in a trauma center, and say don't get one at all. I've seen far too many life altering head injuries, spinal injuries, amputations, and death from ATV accidents, especially ATV's versus trees, fences, guidewires, rollovers, and ejections. There isn't a week that goes by that an ATV injury doesn't pull into the ER. Yes, I know, I'm the proverbial party pooper. If you decide to get one anyway, be sure they have top of the line helmets, protective gear, and ride in a cleared area. Sorry I'm so gloom and doom on the subject, because I do understand the fun and attraction of them.
I won't gainsay the posters who have been pushing a 16-yr age limit. Let me just say that I started riding on a "boonie bike" with a 5-horse Tecumseh engine at age 9. My father kept me in the yard of his country house tooling around the driveway loop probably for a year, then let me out on local trails, but only within a certain area for another year. After that, I was freed up to ride pretty much where I wanted, and that was on 2 wheels. If you get the right-sized machine, and make sure they know what they are doing, you can probably feel ok about letting them out on your property at 13. That means no Yamaha Banshee or Raptor/Polaris Predator or other massive racing machine; it means they both take a riding course (if you happen to buy from a dealer, even a used machine, this course is generally free), and you spend some time watching them ride somewhere where you can keep track of them so you can judge their abilities. Granted, thirteen-year-old boys have an almost infinite capacity for becoming total idiots when outside parental control, but if you drum the fundamentals in enough, at least some of it might stick.
Bombardier makes all of John Deere's 4 wheel ATV's. My wife's machine is a Bombardier Quest - the same as the JD "Buck". It is a fine machine - it has a very low, wide profile with a low center of gravity that gives it excellent stability.
Two problems: weight and cost. It weighs over 600lbs. dry and isn't cheap. On the other hand, the build quality is superb (which is where the weight comes from) and properly cared for it will last a long time.
The 500cc model has plenty of power so don't waste $$ on the 650cc. There is only a couple of mph difference in the top speeds between the two. I believe it has a 6.4 gallon fuel capacity so it will go a long time between fill-ups - especially with that fantastic Rotax 500cc motor. The belt drive tranny engages quickly and is very quiet. It starts easily, even when cold.
Two-up machines are somewhat controversial. They may actually be "semi-illegal" in some states which prohibit double riders. Don't know about WI.
MAKE SURE YOUR BOYS WEAR WRAP-AROUND HELMETS!!! NO OPEN-FACE STYLES! I work in the medical profession. You don't want to see what happens to unprotected heads and faces in a crash. I dumped a 6-wheeler loaded with rocks last summer and of course I landed on the rocks which spilled out of the cargo bed. Those Idaho rocks are HARD! My back and legs were bruised and scraped. My helmet had two deep gouges on the back. That could have been my head but it wasn't. Needless to say, I haul rocks in a trailer now...
If you would like to hear more about my experiences with this ATV, contact me off-line.
And machines are much more tolerant to kinetic energy in the magnitudes we're talking about here than bone, flesh and sinew.