Statch off the paint draw your picture revealing the hidden colours beneath
Card, Wax Crayons, Black Paint, Cocktail sticks
I would suggest you pre-make the scratch card before your meeting, unless you make this a 2 week activity.
To make the scratch cards, take a piece of card and cover in a thick layer of wax crayons using various designs. Once this is done, cover the whole thing in a thick layer of black paint. I would suggest doing multiple layers of black paint otherwise it takes forever to dry :-)
Once dry take a cocktail stick and scratch off the paint to create your drawing revealing the colours beneath. See picture