I find that ear plugs don't work well for younger kids. The shape of their ear canals usually just works them right back out.
Pletor, Radians, "Field & Stream" brand, Remington, and other companies all make youth sizes of hearing protection that will fit your kids' heads a lot better and provide much better noise reduction than adult-sized muffs.
Sometimes they even come packaged with youth-sized safety glasses -- which are another MUST.
It is not a good feeling to turn around and see your young child standing on the firing line with his too-big muffs half off and his adult-sized glasses slipping down off his nose -- while you're banging away with a firearm a few feet from his/her head.