“There is something comical, and maybe silly, about relying on kids,” said Paul Romer, a professor of economics at the Stanford Graduate School of Business. “It seems risky.”

But Mr. Romer noted that it was getting tougher to pick the winners among start-ups. Young people, Mr. Romer said, may be better equipped than investors, who tend to be in their 30s or older, to see nuances and identify trends.

“The people making the decisions may not appreciate some of the small differences that might be apparent to end users,” he said.

Those end users include Mariana and Tatiana Megevand, who live in Geneva. Last year, Neil Rimer, their uncle, heard the girls, 14 and 12, talking about one of their favorite Web sites, Stardoll.com.

The site lets visitors create and dress up virtual paper dolls. Mr. Rimer is not just an uncle but also a venture capitalist, a partner with Index Ventures, based in Geneva. He decided that his nieces’ interest constituted one of the better tips he had heard in a while.

“The next Monday I went in and talked about it with my partners,” he said, “and that week we were on the phone to the company.”

Index and other firms, including the venerable Sequoia Capital, have invested more than $10 million in Stardoll this year, and the company has moved to Geneva from Finland. Mr. Rimer says he still talks to the girls about what they like and what they would improve. He has given them some incentive, too: a small stake in Stardoll that could be valuable if the company prospers.