We did these in fourth grade, but I think third grade at least could do it. Get some of those "keyhole" pavers and paint them white. You have to prime them first, if you want perfectly white snowmen. We didn't prime, and our snowman have a more "rustic white" look.
Then you can use "paint markers" to paint two eyes and a mouth. Use an orange button for the nose. Use black buttons for the, well, buttons. Strips of flannel fabric can be used to tie around the neck for a scarf. Big pompoms and a pipecleaner can be glued on the head for "earmuffs."
They are supercute when finished and very fast if they're pre-painted white.