As the passport holder when we travel, this is a good idea. My cross-body purse is safer than anybody's backpack, back pocket or shirt pocket as it has inner zipped compartments, and I always keep passports in there. Every time we needed to show them (to board a plane, to get on a ferry, to register at hotels, to enter certain museums and attractions - all stuff we did recently) I had to open them to see who they belonged to. Some places require everyone to show their own passports, regardless of age. A color coded post-it note would work as well.