The AR-15 is the ULTIMATE (IMHO) kid magnet when at the range...with so little felt recoil and it’s obvious COOL look, the gun is easy to get hooked on!
Of course, what kid doesn't want to shoot a firearm that looks like the ones carried by the HEROS he sees on the news--our Soldiers in Iraq!
Even an older 3 or 4 position CAR stock will work well for this purpose.
But also, with kids, I prefer not to use a 20" heavy aka match barrel either. Arm strength is a factor, so a 16" is better, and best a 16" pencil aka featherweight or whatever you want to call it, as light as possible.
Introducing a kid to shooting FIRST with a .22LR is preferable. A 10-22 is a great confidence builder. Ditto handguns, a .22 LR is the way to go for this.
Make sure the rifle, shotgun, or pistol you give a child, or newbie, FITS. There is no need for a child to have to tuck the stock under his armpit. Buy, borrow, modify, whatever, obtain a firearm that fits the person.
I have never understood handing a small kid a large bore rifle or magnum handgun, just to watch him get knocked on his butt so the adults could all have a good laugh. We have all seen this, and probably had it happen as a kid. This is stupid. Should you see adults about to engage in perpetrating this cruel joke on a kid or young lady, stop them. This is not a "right of passage" for a kid, it is just sick.(Read in the General section about the kid KILLED by the recoil of a .454 Cassull, boucing back and hitting him in the head.)
Eye and ear protection are a must, even with a .22. Kids will have difficulty with full size muffs, so help them with disposable foam rubber plugs. I keep a double handful in my range bag for visitors and guests, and half a dozen SPARE safety glasses new in wrapper. Even if they are not with me, just onlookers, I offer earplugs. BTW, most foam disposable earplugs EXCEED the noise reduction rating of most muffs. Muffs average -24 db rating, foam earplugs are in the -31 db to -33 db range. That is a very great difference. It does not help to put muffs over plugs, little additional noise reduction is obtained.
Go over all the safety rules. KISS.
Before handing a firearm to a newbie, with an empty firearm, show him how it functions, how to maniputlate the safety, mag release, trigger, etc. Get the person comfortable through dry fire with being able to operate everything without looking.
Then proceed to actual firing of the firearm.