There is significant sound transmitted to the ear canal via the bones around the ear. Muffs are needed to help deaden that sound. It's not a bad idea to use plugs too, but if you're going to use only one method, muffs are probably the way to go..
Be sure that the muffs seal all the way around their perimeter. Remember that the child won't know how the muffs are supposed to fit--you're going to have to help them make sure they are fitting properly and working well.