OP, you need to fact find, as said.
I've seen groups of Nursery and School children that have been allowed to get over boisterous and there's been bumped heads etc, with correct control, it wouldn't have happened.
Likewise, at this point, no-one knows what the flooring was, under the play equipment.
Focus on getting your DD back to school, read the accident report and see what the conclusion is. Then consider if you have any further questions.
When I was a child, there were lots of accidents, people didn't accept the findings and things got safer. A bit like not accepting child injuries in a Car, so seats were introduced.
This might have been an genuine accident, there might be things that could have been done to prevent it. I'd put that on the 'to do' list, rather than worry about it, this week.