If they're young kids, hard to beat blow-up balloons staked to the ground. A bit of a pain to set up, a bigger pain to CLEAN up...but boy do kids like it. Easy to hit, colorful, usually moving a little, nice "pop" sound, etc.
Little kids (in my experience) enjoy a bit of shooting then usually want to move on before long. Balloons? Pretty much will shoot those as long as you're prepared to blow them up, set them up. I used to wear out before the kids did!
