'My experience has taught me that often when people do you a favour financially, it’s always done in the expectation of payback in some way. '
Only if the person is a twat.
And to all the people who say 'just pay her', the OP doesn't have the money!
She also has a mother like mighty, which is sad.
Tell her you can't pay her, because you can't. That you paid for the meal out with vouchers given as a gift.
Don't offer her a penny because the only excuse for doing this to your kid and grandkids is that you're a total cunt. Who the hell doesn't want to help their child and grandchildren when you know they're doing their best but struggling?
'Sorry, we don't have the money to pay you.' And then I wouldn't bother contacting her again. Seriously.
I cannot imagine doing this to my children and grandchildren even if I were completely impoverished. I'd have taken a coach over (cheaper) to help my children and grandchildren if they were struggling, too.