Neighbour has offered to keep my class 2 son after school till I'm back. How do I make it fair to her?

His school van drops him at 3.15. I finish at the clinic by 6.30 most days, and my partner is later with extra classes at school. We've been patching those hours with a sitter who has now found full-time work. The mother of his friend downstairs has offered to have him at theirs till I'm home. The boys are thrilled. She says she doesn't want money, and I believe she means it, but three hours a day, five days a week, is a lot to accept as a favour. How have others set this up with a neighbour? Paid anyway, shared the cooking, took her kid on weekends? I don't want to lose a good neighbour over it 🙂