The OP’s grandchild is 5. A lot of the recommendations here are more for pre-school children, surely? I’m never sure what my eldest grandsons will like now at 7, 6 and 4, so pick up a few books at charity shops when I see them. The Wimpy Kid, that multi-storey treehouse series, Horrible Histories, Roald Dahl, David Williams, Horrid Henry for the youngest. They all like factual stuff as well - football, pirates, kings and battles, how sewers work. I dislike a lot of the above, but they like them, which is what matters!
I’ve tried offering the updated Enid Blytons and they’ve been met with a total lack of enthusiasm. I’m quite sad about that, as I used to love The Faraway Tree, The Wishing Chair and the school series, and so did my children.