Give her some money to take herself and the cousins out for lunch after swimming lessons (swimming uses a lot of calories) - just in case the kids are "too hungry to wait". Or the idea of having an outing for the four of you (Grandma, your kid, the cousin, and yourself) that costs "about" the same as the swimming pool fees might be fun, too.
Take photos and give Grandma a few, possibly in a digital frame, so that she can display them.