I bought my son the Henry Mini Bolt
http://www.henryrepeating.com/rifle-minibolt.cfm when he was 5 years old. It's been the perfect training rifle for him and his younger sister. Buds has them for $191.00 & free shipping
http://www.budsgunshop.com/catalog/product_info.php/cPath/36_107_979/products_id/36682
I wanted to start my kids out as young as possible with single shot .22 rimfire for safety. I bought only one rifle because while one kid is learning to shoot the other is learning to spot and act as a range safety officer for the shooter.
When teaching children to shoot I work only one on one. and I am never further than arms length from an active shooter. Too much can happen in a split second of inattention for me to feel comfortable with multiple shooters until they are much more mature and experienced.
My advice is to start with a mini sized single shot rifle with iron sights. That will be plenty exiting especially with reactive targets (ice cubes, animal crackers, tennis balls etc). Later on, once they learn the basics of marksmanship and safety, you can introduce the semi autos and military style rifles. Otherwise they wont learn to shoot they will just learn to miss really fast.
The size of this type of rifle allows very young/small shooters to use it from standing, prone, kneeling and sitting positions. They are not tied to a bench and rest as they are when it's a gun they will need to "grow into" in a few years.
I picked the Henry because it was small, light, accurate, stainless steel/synthetic construction, fiber optic sights and most important of all it has a separate safety. Unlike the other single shot .22 rifles that rely on a cumbersome cocking piece as the only safety, the Mini Bolt has the ability to be carried chamber loaded cocked and safety on just like any other hunting rifle.
Over the past 12 years that little gun has been shot at least ten thousand times by not just my 2 kids and their friends but by several scout troops as well. It has held up just fine.
My son is now 17 and has graduated to a CZ 550 American as his .22 bolt action. He also has access to top end .22 rimfire ARs, and a customized 10/22. Every now and then he still asks to bring the Mini Bolt to the range as it is just so much fun to shoot that little rifle!