What would you want?

Im setting up my home for foster kids and I read one thing to help boost trust with your kiddos is give them a space that is just theirs, like a locked box or something, so they know for sure some things (like a diary etc) is definitely not being looked at. Ive looked into a couple options. What do you think sounds best.

OPTION 1: A locker bay that can go in the mudroom or something like that. works since I'll also need a locker for medications so it can serve that purpose too (each locker has different keys, they're not able to be opened by any key you need the specific one for your box)

OPTION 2: Individual locker that they can keep in their room (like under their desk)

OPTION 3: Locked backpack. Well be getting them a backpack regardless, so this could be killing 2 birds with 1 stone, but if part of their wanting for privacy is a nosy foster sibling this could easily be cut open.