Cool... I don't want to spend a fortune on this...

Since I'll have the kids tie the squares together, what should I use? Yarn? Or fabric ties?

Yarn, crochet thread - the stuff people crochet doilies with, perle cotton, embroidery thread, 1/8 inch ribbon, are all things people have used to tie quilts.

You need needles with eyes large enough to accommodate whatever you decide to use, with a sharp point.

I'm sewing it on the squares, and then using that to tie the squares together (I won't have a sewing machine and I won't have time to hand sew a thing, so I have to tie this together.)

Are these squares going to have raw/unfinished edges?

Why cut up the sheet (or large piece of fabric) at all? Just draw squares on it and let them do their thing. You can still tie it (treat it like cheater cloth). I've dealt with hundreds of things like this and find the simpler the better.

You might draw up a letter requesting donation of such materials if you have a letter of Non-profit statement (check with the director of this camp group for a NPO number if required by your state for tax exemptions). A lot of places will donate, reduce prices, etc. I've done this a lot. You might be surprised by what you find.

Lab fairy, the no cutting is a fantastic idea... saves me time! :) I'll go get the flat sheet from AJ Wrights (discount store) They have wal-mart sheets there for 3.00 so I'll get that and a sheet to go with the flat sheet I just bought there too!

Awesomely simple/easy idea!

IT IS! It saves me some time!!!