To be clear, it wasn't machinery the kid was actually stuck in, it makes it seem like he was caught in some big machine. He stuck his head through the ceiling rose and when the pole moved it pinned his head against the fibreglass opening on the ceiling. He wasn't just standing up on a character when riding, but literally climbing up the character and standing on the seat itself, sticking your head up against the pole. It's not a fast moving ride in the slightest, normally you would expect someone would see the roof coming close to them and duck down, not stick your head through the opening.
There is absolutely no way to enclose the hole the poles come through, you have forward/backward motion of the crankshaft, plus some lateral movement due to alignments that have to be allowed for it to function. As said, they normally install this nylon brush stuff as a deterrent, but it's not something that actually stops you from sticking a hand (or other body part) through the ceiling.