'Ears' are important
Definitely recommend going with muffs for kids - their hearing is so much more sensitive than adults. I second the notes above about using foam plugs along with the muffs for younger kids and that kids rarely know how the muffs are supposed to fit.
I have 6, 8 and 12-yr-olds. The 6 year old uses muffs w/foam plugs, the 8 yr-old uses the peltor youth (~$25) muffs that he says are comfortable enough to wear all day, and the 12-yr-old uses a set of shotgunner muffs (slimmer, lighter, tighter fit for him - he is in between the youth and adult sizes).
Another thing we watch for is when they put on their 'eyes' - sometimes thick frames will unseal the muff.
Kudos for introducing the next generation to the fun and safe world of shooting!