Halloween Costumes - New Athens,IL

If you are able to draw/paint the characters, you could buy one or two (depending on the sizes needed) of those egg-carton-foam mattress things that are sold to put on your bed (not sure exactly what they're called).
Cut it into the correct shapes (rectangle and starfish), and paint the flat side to be the front. We usually use spray paint for the main color and acrylics or permanent markers for details. You can also add embellishments, such as Spongebob's tie, using cloth sewn or glued on.
We did this to make a banana costume for one of my sons last year. We just cut a hole to fit around his face: snugly so it would stay on, but not too tight. Fortunately the foam is somewhat stretchy and it's easy to cut with scissors, so it wasn't hard to get a comfortable fit. We didn't even have to secure it with anything else.
It was easy to wear, could be taken off and put back on in seconds, and was light and very flexible (no difficulty sitting down or bending over!). Not to mention, it was relatively inexpensive--and he got tons of compliments! ;-)
You could probably do the same kind of thing with large sheets of craft foam if you can find them... or you could use cardboard, although that wouldn't be as flexible and would need a way to be secured (tie-on with yarn?).
HTH! Good luck, and I hope they have fun!
--A.

I have 20 years experience in making homemade costumes, and let me recommend going to GoodWill or the Salvation Army stores for your items to alter into other costumes. Often we just take whole clothing items and do some alterations and wha-la, a great halloween costume. One of my daughter's favorites was the straight jacket I made for her out of an overlarge men's longsleeve shirt and white belts. With the help of makeup and hair gel, she looked pretty scarey!
I made a Ninja Turtle shell out of paper machette and used Goodwill tights and green sweats.
You could go with the lightweight foam for the Spongebob and possibly spray paint it yellow, and get yellow sweats, or white sweats and get yellow RIT Dye @ WalMart. (You can find the tie and shorts if you want to get really detailed)
For Patrick I'd say try oversized sweats that are the same color as the character and stuff it with pillow stuffing because he's kinda big and shapeless.

As the other two ladies reccomended, try buying large pieces of foam. Or using cardboard. Make the outfits they wear yourself and sew or glue them on... You can also always buy the costumes online through ebay or something.

It might work if you got a sheet of foam and cut two star shapes and a also made a rectangle large enough to fit your boys. You could slit the back so they could get in (probably leave openings for their feet). Make the shorts, tie, etc and sew to the costumes. Hope this makes sense -- I can see it in my mind's eye.

S.