This is probably one of the most inexpensive pieces of artwork I've ever created. I always save corrugated cardboard to reuse later. It's sturdy and has one smooth side, making it ideal for all kinds of projects. I was in the mood to paint, but was all out of 8 x 10 canvas, so I raided my cardboard stash.
I used an XACTO knife to cut the cardboard down to size (using the glass panel that came with the frame as my template) and painted one of my simple, whimsical cat paintings. *See the original ACEO here. The frame was a resale shop find (2 identical frames for one dollar!). The saying "Does this sweater make me look fat?" was printed on my computer. Voila! Instant original artwork!
I used artist acrylics, but you could easily do this with kids using inexpensive tempera paints or even markers. Just remember to save all those free sheets of cardboard that come with all kinds of things!