Start with a box of sandwich cookies, like Snackwell's. To make icing, use mixer on low speed to beat together 1/2 box (16 oz.) confectioners' sugar, 1 Tbs. powdered egg whites, and 4 Tbs. water until combined.
Increase speed to high and beat for 4 minutes. Thin icing with 1 Tbs. water until it has the consistency of heavy cream, and spread it over tops of cookies with your finger.
Once the icing's dry, have your kids write loving messages with edible food markers.