The sandbox question I can't answer. I've also never seen (or heard of) that movie. The idea came from my kids playing under the covers.
One of the things for the test logs that I was planning (if I decide to move forward with this) is for things like feet to appear out the side to lure people into trying to help pull them out. Of course, at that point they are pulled in. Like soulless said, however, it is just a bed that eats people. Not exactly groundbreaking stuff but I was going more for a "thing that we all have in our homes that now creeps me out" vibe rather than a "eldritch horror from beyond space and time" thing.