Going back to caveman and cavewoman status doesn't mean you have to forfeit style and comfort. These costumes use clothes you already have or old ones you don't mind tearing up!
What you need to do: For the girls, wear a green top and black shorts. For the guys, wear a grey shirt, tie an orange shirt (with scalloped ends) around the waist, and wear shorts. Get black felt, cut out the shapes, and stick them to your clothes using a hot glue gun or tape.