This is what we did! ( great for ages 3 to 8 year olds!)
We used some old baby socks, white for the body,and coloured ones for the hat! We filled our snowmen with cotton, but I have seen a very nice version with rice. It makes them more stable! We used thread to tie the sock, and then some leftover material to make a scarf. Some old buttons, little paper pompons and felt pens did the rest! They really enjoyed it and even took them to school to show their friends!