That is most definitely illegal.
what law does it break ?
the sign on them that says "parent and child only". Well thats contract law, implying the person that enters the carpark enters into a contract to obey the rules.
However, the ticket is left on the car, which if not paid, will be sent in writing to the legal keeper of the vehicle.
However, how do you know that the legal keeper of the vehicle entered into said contract ? you have none.
The same legal precedent that the keeper is automatically liable only applies to council parking fines and speeding tickets. Contract law is not enforcable by these means.
Do some research first before making wild inaccurate statements.