My initial thought on this would be to put a tarp (or something similar) down in the area you are going to put the scene. You can cut the tarp in an irregular shape, if needed. Get enough bags of play sand or leveling sand from your local hardware store (Lowe's, Home Depot, etc) to cover the tarp. Arrange various arms, legs, etc. in the sand to give the appearance of sinking victims. You can make the bed of "quicksand" look more natural by arranging plants, rocks, or other materials around the edges.