Wet Magic by Edith Nesbit, Fiction, Fantasy & Magic in Bloomington, MN

And when that mermaid is captured and put on display at a local circus, they decide they must rescue her.
As their reward, they are permitted to visit the hidden kingdom of the mer-people, but find they must now stop a war to save their new friends. "That going to the seaside was the very beginning of everything,- only it seemed as though it were going to be a beginning without an end, like the roads on the Sussex downs which look like roads and then look like paths, and then turn into sheep-tracks, and then are just grass and furze bushes and tottergrass and harebells and rabbits and chalk."