It's 10:30 on a Saturday morning, and like a typical New Yorker, Biz Johnson-Brown, 31/2, is running late. Class has already been in session for half an hour when she parks her purple Princess bike near the door and, accompanied by her father, Bruce Brown, joins half a dozen other children as they climb and crawl and tiptoe their way through a miniature obstacle course under the relentlessly cheerful supervision of their beloved drill sergeant, Miss Leah.
This is the big-city version of Gymboree, the international kiddie-fitness franchise, and if you didn't know you were in the windowless basement of an...