Wash the milk carton thoroughly with very warm soapy water. Make sure to remove all traces of the milk. Rinse the carton and use a clean dish towel to dry it thoroughly.
Apply a little glue to the opening of the carton, and close the flaps. Clip a clothespin onto the closed flaps. Let the glue dry, and then secure the flaps with staples. The glue will prevent water from seeping into the birdhouse when it rains or snows, and the staples will keep it securely closed.
Tear off pieces of masking tape and use them like wallpaper to cover the entire milk carton. Make the birdhouse look like rough bark by covering all the tape with a coat of brown acrylic paint, or apply a thin layer of brown shoe polish to all its surfaces with a soft cloth. Allow the birdhouse to dry thoroughly.
Ask the kids to add more decoration, if they want. Make a perch by cutting about 3 inches from the end of a drinking straw. Paint the perch brown, and let it dry. Poke a hole about ½ inch from the bottom of the doorway, and stick about ¼ inch of the perch into the hole. Seal around it with white glue.
Spray a coat of clear sealant over the entire birdhouse, and let it dry thoroughly. Poke a hole through the top flaps of the birdhouse. Run some wire through the hole and use it to hang your birdhouse from a high tree branch in your yard in the fall.