We aren't in the UK, but older two dc were charging a minimum £5/hour when we lived in a compound, up until a year ago. It was understood that anything over midnight, or a later return than originally agreed, would be generously rounded up. They'd have been 14 & 16 at this point.
They were kept busy every weekend, so would prioritise the houses where they knew there'd be a pizza delivery laid on or generous snacks.
Actually they used to compete to babysit for the Deputy Head, who could be relied on to arrive home late & spectacularly wellied & over pay by emptying his wallet in their general direction!
Popular gigs were also walking distance ones - getting an uber was a minor pain regardless of whether it was paid.
They'd just be politely unavailable for future bookings where they'd been even slightly cheapskated on payment, or the kids were a nightmare, or getting home was a nuisance.
It was very much a seller's market.
So yes, your dc has been taken for a mug on this occasion. Lesson learned - not available to sit for these tight buggers again.