Your Granddaughter has done the work, so can expect to be paid. If the pub decided to give her cash in hand,they cannot reasonably expect that money to be paid back at a later date.
They might have asked for a receipt at the time of payment (but presumably didn’t) or they retrospectively may ask her to provide an invoice for her work done (but she doesn’t have to provide one).
The pub have nil prospect of recovering this money, because there is no paperwork to prove your Grand Daughter was given it. She is not compelled to give it back, and the pub cannot take her to Court for none payment.
A bit of a mess, and pretty amateur business on the pubs part.