I take kids shooting whenever I can. It's interesting to see what they gravitate to, I usually bring 3-6 guns and we shoot them all, then go back to their favorite some more.
Muzzle loaders are usually a big hit. The mechanics of it are cool, plus fire and smoke come out, and you can do super light loads for them to start out. Also non-gun parents are usually more willing to let their kids try that (it's historical, not so scary, I guess).
Otherwise the Remington 597 with a cool Boyd's stock, the Ruger 22 auto, and the AR15 are usually hits with the kids. For smaller shooters the little Rossi youth size 22 single shot is great- has fiber optic sights and it's tiny and light.
For brand new shooters I also keep a stock of Super Colibri ammo, it's so quiet you can hear it impact on the cardboard targets!
Always eyes and ears, and close targets on big cardboard sheets. Cut out their targets, write down the date and the gun with a sharpie and send it home with them so they have a story to tell their family and friends.