The idea:A sugar-packet-size GPS that monitors children’s whereabouts

The aha moment:Three years ago, John Renaldi looked away momentarily and his 6-year-old son scampered off while playing at Maggie Daley Park. After 30 minutes of increasingly frantic searching, Renaldi found Ethan, oblivious and happy. But that feeling of panic stuck with him.

Since then:The former Motorola Mobility vice president, 38, used his gadget-intensive background to create a vigilant parent’s dream come true: a heavily encrypted device that lets you follow your kid’s movements on a smartphone app (yes, we saw that episode of Black Mirror)—the same way you watch your Lyft driver wind through the streets. Renaldi incorporated the company just three months after his Maggie Daley freak-out, bringing aboard a former colleague, Rodger Ady, as cofounder, hiring engineers in Chicago and California, building prototypes, and pitching investors. “The first model was about one-fourth the size of a conference table,” Renaldi says. “And we said, ‘We’re going to take all this and shrink it to the size of a quarter.’ ” The tracker is now small enough to clip on a belt loop, hide in a backpack, or attach to a shoelace. It costs $100, with an annual service contract that runs $8 a month, but it’s been constantly sold out since it began shipping in December. (So far it’s available for purchase only online.) Coming soon: a pet tracker and, possibly, partnerships with several government organizations, though Renaldi can’t say anything more about potential 007-type uses. “That validated the project to us—like, ‘Holy cow, there is something to this,’ ” he says.