Hmmm....this might be too young for that group, but one thing I did with lower elementary was to get an old round pie tin and cut sheets of black paper in a circle that would fit inside. Using white paint and marbles, we marble painted a web.
The "spiders" were made out of egg carton cups with pipe cleaner legs and wiggly eyes.
If you used some other material--maybe sculpey for the head/body and thin wire for the legs--you could still marble paint the webs. I did marble painting (different project) with some 6th graders and they had a blast!