Fill a small bowl with white paint and drop in some marbles. Cut out black construction paper circles. Each child need one. Let kids scoop a couple of marbles out of paint bowl with a spoon and drop them into pie pan. They then roll them around by tilting pan.
White trails look like spider webs.
When dry, give each child a spider sticker to stick in middle. Glow paint also works well but is a bit pricey for more than 10 or so kids. Either bought or made with brown paper bags. If you don't like the hitting, make or buy one with pull strings. Each child must find a pumpkin before he can paint one.