I carried my minor kids' passports almost all the time and presented them to airline check-in desks, along with my wife's. I slowly trained the kids how to present their passports to immigration as teens, and how to use their Global Entry cards as ID (even when they don't have to as minors). When my daughter was 8-ish and I was starting to trust her carrying her passport to the boarding gate to display for international flights, I would collect the passports upon getting to our seats in our plane. Once, by an incredible stroke of luck, she dropped her passport precisely upon crossing the threshold to the plane, and it slipped through the gap and landed on the tarmac. An airline employee quickly retrieved it, but that set our training back a bit.