What are their favourite books, cartoon characters, etc.?
Start with a sweat shirt and pants in appropriate colours, then sew or glue on appropriate trim, or iron on patches. Kinkos can make an iron on patch from a picture you provide.
I loved making my kids' costumes, some were pretty easy, some more difficult.
A long rectangle with a hole for the head cut out in the center can be a knight's tunic (paint or iron on a shield or emblem), put a belt on top and add a toy sword and stick horse.
Turn the tunic into Robin Hood by adding a triangular cap with a feather and a bow and arrow, or into Peter Pan by adding a flickering LED votive light under the edge of his shoulder and sprinkling him with glitter.
Get a dress shirt several sizes too large and put it on so that the collar is on top of his head (use a swim noodle or football shoulder pads to fill in the shoulders of the shirt) and the front buttons are around his face, which you paint to match the shirt. Add a cape on top and give him a plastic pumpkin and a stick horse and you have the Headless Horseman
You can add fur and hoods to make them into Maxx and a Wild Thing. (of just fur and tails and hats/hoods and face paint to turn them into various animals).
Large and colourful shirts and pants can be turned into a pirate outfit.