I'm so excited to have found this website for making home-made pop up cards. The thing I really like about it is the opportunity to incorporate M's artwork in creative and pretty ways.
The website has about 20 different printable templates for pop-up greeting cards. For Mother's Day this year, I printed off the pieces for a pop-up flower and had M color the flowers for her grandmas and great grandma. It really wasn't tricky putting them together either (If it had been at all like origami I would have given up on it :-)
The website has a really cute bird peeking out of a birdhouse card that I think I will try soon. Let me know if you try some of the designs with your little ones (and whether you have any success or ideas for improving them!)