I have been reading your post and it reminds me of an incident that happened with my own son who has severe cerebral palsy when he was 5 years old. He had recently been supplied with a specialist sleep system for his bed to keep him supported properly as he is immobile too but the sleep system was too big for the wooden surround of his cabin bed to fit back on, so we left it off.

One morning we heard a thud & a scream which was him falling head first off a 4ft high bed onto the floor. He has strong muscle spasms in his legs, which allowed him to push himself to the edge of the bed and when he fell he could not put his arms out to save himself. We rushed him to the hospital where they x-rayed him and thankfully there were no fractures but he had concussion.

We managed to get the wooden surround put back on the bed temporarily until a hospital bed was supplied for him. He still has a visible bump on his forehead where he hit and he shows signs of PTSD from it. He is now 23 years old and with (slightly) limited speech he will talk about it.

OP - Try not to be too hard on yourself over this, as there were other factors that were in play at time of the incident when the child you look after was injured. The changing table was too small, there was no side safety rail, no safety straps and there was supposed to be two members of staff with the child at the time.