I have been lucky here as being a registered childminder ofsted have had myself and all my family enhanced CRB checked.
Anyone over the age of 16 who come into regular contact with the children in my care have to be checked and if i have an un checked visitor I am not allowed to leave them alone with the children, even to go make a cup of tea!
I agree that you should make it clear to the customer that you must have at least one other adult in the room with you at all times.
Sarah