Name it: Subway? Sewers? A children's fight club? Best fighter a girl?

Trying to find the name of a book for a friend. She says it's "a kid's book about a secret (sewer system?) tournament style fight club, where the boy character kept getting upstaged by this mysterious "dark knight" character who turned out to be his friend (a girl)... I read this in the 1970s or early '80s. It was about the kids' private lives away from their parents, but if I remember right they HAD parents. They just took their playtime seriously. It may (or may not) have been in Paris, is Judy Bloom-ish, not alternative history, sci fi, or fantasy."

Hi. You might get a better response if you post this over on the Name That Book group....

I have your response here and no response there. :-)