Crowded as a bazaar and cluttered with screens, the trading floor of the New York Stock Exchange is a bubble of overstimulation, not exactly a place you’d want to bring a child. But when Susan and Michael Dell navigated their way through the blinking booths on a recent Wednesday, they were accompanied by, among others, two toddlers, one of whom wore a tiny but impeccable sports jacket. The founder of Dell Computers, 61, and his wife, 60, were there to ring the opening bell, in honor of the