I would bee moving my child and writing a letter demanding a refund if the one week in advance.
From what you have said, the story is simply not believable.
If the incident truly just happened, why was no one able to tell you what happened immediately? Yes. Small children get minor injuries, scratches and bruises all the time. They are learning to walk, to crawl, to play with toys and control their arms/hands etc. they will hurt themselves at times. It's inevitable.
That they couldnt tell you what happened is the issue here. That they appear to have now made something up to cover themselves ...
The lack of incident reports and communication is something you can use to argue your way out of the fees. If they don't write up the incident report, if they cannot tell you immediately what happened to your child, how can you have confidence they are are adequately supervising him?