It's every parent's worst nightmare: You're in a crowded mall, and one minute your child is by your side — the next minute they are gone. Panicked, you're searching in every direction to see where they might be. Each second feels like an eternity until you eventually track them down. It's a feeling John Renaldi knows all too well (he once got separated from his child at Maggie Daley Park in Chicago), and one that he hopes to eliminate with a new wearable device that helps parents keep track of…