I think your question does raise an issue of Health and Safety for your charges and yourself.
Are you the only supervisor on duty each day?
You could raise the issue with your headteacher first and see what happens.
Then you could approach the Chair of Governors in writing, having told the Head of your intention to do so, if you are still anxious.
Next would follow the local Education Authority, after informing the Head.
Are you in a Union, if so consult without delay.
To me your position sounds like an accident waiting to happen, which may have serious consequences for you and your job.
I realise that to complain is a difficult thing to do, but if it is done with control and calmness, and things are put in writing you should be covered if anything , heaven forbid happened to the children in you care or yourself.
Good luck ,and let us know the outcome.