I doubt that you much like my response...
I put 90% of the blame square on your shoulders.
You did not set any parameters at all.
You did not give them any instruction at all.
You did not take charge of the situation or your cars when you saw how they were driving.
You did not require their help on clean up or after run.
You did not require any responsibilty from them at all

You got what you set yourself up for.