L.R.
Long dark green shirt or sweatshirt for a "tunic"; brown pants of any kind; a brown belt for the tunic (or just get a length of brown fabric. Hat can be made from dark green felt from the craft store, just use fabric glue or stitch it a little to hold the shape (see pictures of Robin Hood for shape). Be sure to put a feather in the cap! Maybe some brown fabric for archers' "leather" wrist guards if you want. My daughter was Robin Hood when she was about six! Be sure any bow and "arrows" are safe -- since he's only two I might do a bow but no arrows, even pretend. Have fun.