The moves come at a time when major corporations, app developers and data miners appear to be collecting information about the online activities of millions of young Internet users without their parents’ awareness, children’s advocates say. Some sites and apps have also collected details like children’s photographs or locations of mobile devices; the concern is that the information could be used to identify or locate individual children.

These data-gathering practices are legal. But the development has so alarmed officials at the Federal Trade Commission that the agency is moving to overhaul rules that many experts say have not kept pace with the explosive growth of the Web and innovations like mobile apps. New rules are expected within weeks.

“Today, almost every child has a computer in his pocket and it’s that much harder for parents to monitor what their kids are doing online, who they are interacting with, and what information they are sharing,” says Mary K. Engle, associate director of the advertising practices division at the F.T.C. “The concern is that a lot of this may be going on without anybody’s knowledge.”