This is a UNIX permissions thing.
By default, when a Mac user creates a new document or folder, it is read-write for the owner and read-only for groups and other. However macOS has locked down access to certain folders to read-write for the owner only, no read permissions for groups or others. These folders are Desktop, Documents, Downloads, Library, Movies, Music, Pictures. The one folder that is readable by others is the Public folder.
So, if you open up permissions for a bunch of stuff living inside the Documents folder, no other user can see them. You can open up permissions for the Documents folder.
All normal and admin users on a Mac are part of the "staff" group, so by opening up permissions to group, you allow access to basically any user on that particular Mac.
One way is to use Finder, select the folder in question, then Get Info (right mouse), and go to Sharing and Permissions. macOS also allows you to add permission to specific users.
Another way is to use a UNIX command "chmod 750 Documents" in Terminal; that opens up access to everyone in the same group ("staff"). My UNIX-fu is too rusty to know how to add permissions.
Another option would simply to create a new folder like "Family" in the top level of your home directory and drag the shareable stuff there.