tbull55: I can tell you from experience in dealing with emotionally disturbed children for nearly 30 years that as soon as your children are able to understand the concept of safety and the ramifications of unsafe actions, you can begin teaching them about firearms and safety. The most important aspects of your teaching are the consistency of your approach both to shooting and safety, and your consistent follow-up for both postivie and negative actions. The first thing that will screw up a kids thought process is when he/she has the lattitude to make their own unexperienced and uninformed decisions resulting from an inconsistent parenting/teaching approach. If you remain consistent, level headed and thoughtful about teaching your children anything, you will be a good parent and a safe and thoughtful firearms instructor for your children.
Age is specific to the individual child, but I would think you would want the child to be old enough and physically capable enough to be able to safety and correctly hold and shoot the rifle while under your immediate and hands-on supervision and control.
Hands-on means you are right with your child and immediately able to stop any inadvertant or unsafe action the child may initiate. I had one father tell me that 'hands-on' was what he was doing when he was drinking his coffee 20' behind where his son was shooting the snot out of the target stand frame.
As for choice of rifle - depends on the stature of your children. Shorter arm reaches call for shorter stocks for more comfortable learning and shooting. It is also easier to teach someone who doesn't have to struggle to reach the controls of the equipment he/she is operating. With that in mind, rifles like the Chipmunk, a single shot bolt action 22lr (by Henry
)is always a great first place to start. Being a bolt action and single shot, you can start with 22 shorts and move up from there - those, and others like them are fun guns, yet still deadly. Once your children understand and can practice safe shooting, then you could move them up to a lever action or a bolt with a clip. I would hold on semi-autos until they can no longer hit their targets with the first shot then I would let them have semi-autos. Of course, they may be 60 or 70 years old by then but hay, they can have a lot fo fun before that. LOLOL. HTH. Mikey.