I'm a worrier about things like these. I always imagined that's why the roof in the bathroom is lower and curved. A lot less room for gaining vertical speed. So you'd be temporarily stuck to the ceiling and perhaps a bit dirtied, but less hurt than getting double struck by first the lockers and then the higher ceiling in the aisle.

I think the parent is more worried about the dirtied aspect.

The roof in an airplane bathroom is lower and curved because that is the shape of the airplane.

It's not the worst place actually, you can't pick up too much momentum since the cabin is so small

They should install a gravity switch. A simply contact that uses gravity to either remain normally-open or normally-closed, and then when the opposite is true, automatically flush.

Open question: to what extent does the flush mechanism in airplane toilets rely on the presence of gravity?

It quire clearly uses a pressure differential to quickly suck your shit out of the bowl. I doubt gravity makes much of a difference.