no offense, but IMO, you've waited too long, so you have some catching up to do.
take them shooting first, before showing them the guns at home. they need to get an understanding of the dangers, and nothing will teach that to a kid faster than loud noise and sharp recoil. if it were me, i'd lead off with bigger bore to make the point, then back off and take them through the fundamentals while their ears are still ringing. once the basic safety stuff is out of the way, take them through it live-fire with the .22s. that is the point where fun comes in.
a few points that are important when working with kids--
-make them understand that they aren't going to break the gun. before letting them shoot by themselves, have them handle an unloaded gun to get the feel of it. they should be cautious, but not ginger with it. for rifles, have them shoulder the gun from port arms, until they can handle it with confidence. a controlled draw with pistol does the same thing.
-explain that they have to take command of the gun, or the gun will take command of them. to quote the gunny, "move the rifle around your head, not your head around the rifle."
-let them miss a few times. a lot of people forget this and coach too quickly. allow them to work things out on their own whenever possible. some kids think a miss is a bad thing and get frustrated. hell, half the fun of shooting is hearing the bang and feeling the recoil. let them know that, as long as they are being safe, a miss is no big deal.
-have fun. make it fun for them. a kid's first time shooting is a life-changing event. you can sour a kid forever by not remembering that shooting is fun.
[ETA: as for the unknown friend, tell your kids that if anyone does something that doesn't square with your safety teaching, your kids should politely leave. to drive this point home, put a ball cap on a melon and shoot it with a hollowpoint.]
[edit2: this is directed at your buddy. i kept forgetting that it wasn't you]