I remember being given money by my grandparents when I was much younger than eight, do I can't agree that a child of eight is too young to have pocket money given her.
Your daughter did the right thing, presumably so she would be able to tell her daughter if she asked where the money from you was, that as she said she didn't wamt it. her mother had given you it back.
You surely know you daughter well enough to know whether she intends to tackle her cheeky little daughter about her rudeness, which may well have been unintentional.
In your place, I would not just stop given the grandchildren pocket money when they visit, but I would ask the children's parents whether they would prefer you to stop.
The child's behaviour may have come out of the blue, but she could be reacting to a discussion she had overheard between her parents. So it might be an idea to ask you daughter if she knows why your granddaughter pushed the money away saying she didn't want it.