This is a very rare sitation, in the large majority of schools there will always be at least one additional member of staff still at school when your club finishes at the latest at 16:30pm.
If there is a member of staff or after school club they may offer to sit with the child until the parent arrives, you should only do this if you are leaving the child with another member of staff from the school and there is a clear handover of responsibility for the child.
If however their isn’t, you must remain with that child until someone comes to collect them. You should call your line manager immediately and they will then contact the child’s parents and find out more information and when they are expecting to arrive.
Once you know they are close or on their way, you simply must wait with the child until they are collected. If this behavior continues to happen, please report to your line manager and fill in an incident report form so that they can speak to the school and parent to stop it from happening again.