I have mixed feelings on the success of this craft, but the kids seem to like it well enough, and it has kept them happily, busy for long enough, I decided to go ahead, and share it.
After seeing this basic concept (usually with paint), a number of different places, we decided to give it a go ourselves.
First, the girls colored the plain side of an unfolded cereal box, with rainbowy stripes.
Then, I cut pieces of their artwork, slightly larger than sandwich size, Ziploc bags.
I added some ketchup to the bags, and sealed them. To begin with I used somewhere between 1/4 and 1/2 cup of ketchup per bag, but that ended up being too much. I used ketchup instead of paint, because this time of year, ketchup is really inexpensive. But, children's poster paint would work just as well.
Finally, I taped the bags to the cardboard, and gave them to the children to "draw" on. Basically, you just rub your finger across it to make your drawings, and your whole hand to erase the drawing.
After a little while, we decided there was too much ketchup in our bags, and they were too pillowy, to allow easy drawing.
So, we tried again, this time just on top of a piece of paper, with half the ketchup.
That did seem to work better. If nothing else, it was an interesting process.
It's great to be homeschooler.