Just got introduced to TUX PAINT by an old high school friend.  It's a free download from www.tuxpaint.org.  I've been playing with it for a bit now (yes, the kids are all asleep) and I really think it's better than Kid Pix (for any of you that use that), which is what my school district uses and what I have downloaded here.  There are optional stamps that you can download as well.   The good thing about TuxPaint is that you can set restrictions so that they can not print or save, nor can they navigate away from that program page.  Try it, I'm sure you'll like it.