For Cindy Nooney's 3-year-old twin boys, playing with the Thomas the Train set at their local bookstore in Southern California is a major thrill. Jack and Sam push Thomas, Arthur and friends down the track, they run around the table, jump up and down — and, of course, they squeeeaal. Nooney expects as much in the children's section of the store. But on a recent afternoon, she was surprised by an employee who confronted her, calling her darling Jack a tyrant. "He was a little loud but this is a children's section," says Nooney. "They run a noisy, cavernous bookstore...