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.
Last year in part of our haunt we did a playground scene for the kiddie area, where nothing would pop out and scare them. We had little zombie babies on swings and on a jungle gym and some playing in a quicksand box. I made a square wooden frame, laid a black plastic tablecloth under it, dumped in some sand and had the dolls sticking halfway out of it. Kinda cheesy I know but it was a big hit.