The coffee table top is GONNA get ruined anyways! :) Repaint a coffee table with chalk board paint. Draw a road, game boards, self portraits, work out homework problems, or just let the kids go wild. It will keep them entertained for hours.
An app that photographs and archives kids' art, and then you can print a coffee table book of the collection. But forget kids. I'm using this for myself. Coffee table book full of my cats, here I come!