Post the majority of your targets at 10 to 15 paces. The best targets are big, close, make noise, explode, or can be taken home. Also try playing cards stapled close together. If you can hit the backstop, you'll get a card as a keepsake. I suggest a Pinochle deck with lots of royalty.
Clay pigeons are also a good choice.
Use push pins to hang them on a chunk of wood. If you decide to use gongs, place them out at about 20 yards away and angle them so that the bullet is deflected to the side. Use at least two sizes of targets, bigger ones for a try and smaller ones for a challenge. If you run a competition, feel free to let each child shoot at the gong that best meets his or her chances for success. The older kids want the challenge, and the younger kids need the confidence.