This project is easy and inexpensive and certainly one in which the kids can help. We took a thrift-store frame, wrapped the inside with yarn and cotton to make it look like a spider web. Then spiders and a "welcome" message were hung within the webbing.
On the back of the frame, along the edge, hammer in the small wire nails. Insert the nails so only about a 1/4" is sticking out of the frame.
Place the felt pads on the back of the frame along the corners so the nails will not scratch the house or door.