Congratulations! We're finding out the sex of our twins, due in late June, this Thursday.
Don't care if it's boy/gir, boy/boy, or girl/girl as long as at least one of them has some interest in going to the range with ol' Dad now and again... So I've had some thoughts on how I'm going to organize exposure to shooting as well.
I'd start him on a 6mm Airsoft BB gun as soon as he was able to fire it, (with you helping hold it, etc. remember Eye Safety!) shoot paper boxes and cups in the garage or basment with a loose hanging blanket for a backstop. Start stressing the four rules as soon as he's able to speak in sentences. Then progress to an airgun, (If you have property or a basment large enough to permit it.) and once he displays complete proficiency and adherence to the rules, I'd get him one of the youth .22's like the Crickett, Henry Mini-Bolt, or even one of the cheap Romanian .22 milsurp trainers and lop a few inches off the stock, saving the chunk for later when he grows.
I'd "gun proof" the kids by having a 3 pronged approach.
- A gunsafe, or at minimum, a Homak style gun cabinet. No one but you knows the combination, (make it something with a combo if at all possible, as keys can be stolen by a curious teenager) if the wife is not interested in operating the safe, leave a copy of the number with a trusted relative (make sure there's no notes as to who has the safe or where it is, should a burglar riffle through that relatives belongings.) If you can only afford a key type lockbox, keep the key on your keyring only, so it's allways with you, and leave the spare with the relative.
- Electronic gunvault pistol box for the bome defense piece that's left loaded. A police cruiser style shotgun lock/clamp if you plan on keeping a long-gun for home defense as well.
- Tell the child repeatedly you'll show him, and let him handle any firearm anytime he wishes, all he has to do is ask you. This will remove any mystery or "forbidden fruit aspect" from them. If Jr. wants to have you open the safe once a day to fondle your AR, or 1911 etc. every day for a month, do it for him. If he asks you when it's not possible i.e. out for dinner, at bedtime, remember his request and do it the next day.
Once he's progressed to the .22, keep in mind that children have a very different sense of time than adults do. An hour car-ride, or even fifteen minutes of watching you shoot while he just sits there can seem literally like days to a kid. If you take him shooting,
you're taking him shooting, not both of you. Spend your time watching and coaching him. Start out using reactive targets like baloons, tin cans, old fruit, or metal plate spinners, to add excitment. Start out with targets close at 7 yards then slowly move them out to build his confidence.
If you need to practice your own marksmanship, test out another gun etc. make a separate trip to the range for yourself. Focus on the child and his shooting. As soon as boredom sets in with him, pack up and leave. If this means you drive an hour to the range, or the backwoods family farm etc, only to have him shoot 50 rounds before he's done, so be it. I probably wouldn't start shooting my own firearms alongside him while he shoots independantly until he's 10 years old or so, unless he was participating in some kind of organized youth shooting event.
As to noise damage from a gunshot or three in a home defense scenario, I don't think there's anything you can do about it. Unless a baby or toddler is within a few feet of the muzzle, the hearing damage is unlikely to be permanant, and certainly worth it to eliminate a threat. I doubt you'd have time to do anything about protecting his hearing, much less your own, if there was a situation.