Financial apps designed for kids are booming. How do they work?

When Ashley Millwood’s phone kept buzzing, she knew there was a problem.

Her two teenage daughters were spending the afternoon with a group of friends, including one whose mother was supposed to have been the driver. Soon after the trip began, the Greenlight banking app on Millwood’s phone started sending alerts: The car was repeatedly going between 20 and 30 miles over the speed limit. When she finally called, she discovered the driver was not a parent, but a friend of the group who had only his learner’s permit and should not have been driving without an adult.