All you need is paper, stamps and scissors....and maybe those tiny fingernail scissors, if you have some.
Just have the kids fold their cardstock. Then unfold and lay flat, right-side up. Next, let them choose a stamp, and decorate the front of the card, with one or more images going above the fold-line.
Older kids can then carefully cut around the part of their stamped image that goes above the fold line. Younger kids will need some help with this part.
I'm much more comfortable with the quick-and-crafty elementary-age approach!