How to build a sinkhole simulation?
I would use sand and experiment with different size openings under different thicknesses of the sand. If you fill a small container with a couple inches of sand and then open a hole on the bottom of the container it should simulate a sink hole. 

Start with a funnel full of sand and see if it flows good when you take your finger off the end of it. Maybe try salt or sugar for their flow characteristics.

Good luck with your project!