I grew up in a time before cell phones, I had a lot more freedom than your kids will ever experience. Crime rates were much higher back then by the way. I still survived even though my parents rarely knew where I was between 4 and 6 pm, but I was always home in time for dinner.
I suppose the truecrypt successor veracrypt would work well for this, it is available on all 3 platforms which probably makes it stand out from most other solutions.
For my generation it’s close to universal