I read this in the early 2000s, I think. It was a paperback, with the main characters being a set of kids, at least three of them, with both boys and girls in the group. I don't remember the main part of the narrative, but I remember it involved them having an adventure or solving a mystery. There is an older lady who had a cool house with a number of button-invoked amenities. At the end of the book, as the kids are waiting for their parents to pick them up, or drive them home, they push the button marked "Pool", expecting to play billiards, only to have the floor open up into a swimming pool, to which one of the comments that they don't have swimsuits with them.