Here's a specialized niche for you: perspiring 7-to-10-year-olds. That's the market Bertsherm Products Inc., of Cleveland, is going after with its new, all-natural kids' deodorant, Fun 'n Fresh. Executive vice-president Stephanie Wood, a 10-year veteran of Gillette Co., says that big companies think the teenybopper niche is too teeny to pursue. She thinks they've miscalculated, noting that 34% of preteens already use deodorant or antiperspirant. Fun 'n Fresh is projecting sales of $19 million by 1990. And you probably didn't even know that kids sweat. -- Leslie Brokaw