As hard as it may feel to do, I would suggest that you try to put the accident behind you, and focus on your son.
Accident's happen and although this happened while the member of staff was moving a sofa, it could quite easily still have happen if she had just been standing beside the sofa and stepped back.
You will find that if the Nursery has a strong safety focus that their risk assessment has already been updated, to try and ensure that this does not happen again, I am afraid that risk assessments alone will not stop accidents for occurring.
Sparing a thought for the member of staff, I am sure they feel at rock bottom, and helpless to do anything to improve the situation. I am speaking from experience, as a couple of years ago, one morning a child came in to the room to be dropped off, and as she ran over to hug me round the legs from behind, I stepped away not knowing she was there to attend to another child who had decided to try and climb onto a table. The child ended up running head first into the wall her dad who hadn't even signed her in yet, took her straight to A&E (he was a doctor) and it turned out that she had fractured her skull. She was back in the Nursery 2 days later and every time I saw her complete with bump, stitches and bandage my heart sank.
I know that no amount of risk assessments would have stopped that from happening, and even phoned and asked our early years adviser to call in to review the report to see if there was anything I could have in place which would have prevented this.
The child's parent's were great about it, and we ended up having a stronger relation because of it. I have moved on from that Nursery now, and was even sent a photo of the child in her new school uniform when she started school.
I guess this is the long way of saying that if you are still confident in the care the nursery is providing for your son, try and put the accident behind you.