It's like an 8-inch-square hole. Big enough for a shirt or pants or a towel to slide down, maybe with a nudge from a foot. By the time a kid can open the closet door to get to the hole, they'd have a hard time fitting through it. Any cat that falls through a hole in the floor has only itself to blame, and can likely manage the problem just fine.
We have kids and a cat and none have had trouble with it. It's not half as dangerous as and has a lower attractive-nuisance factor than any set of stairs.