why not include that (encrypted) file on the drives themselves?

That is what I was trying to convey. Both unencrypted instructions for where to get 7-zip, the encrypted file, and hints to what "family password" means, replicated to multiple thumb drives in the event one fails or gets corrupted. If they are using windows they may not even need to download 7-zip but one never knows.

    D:\ dir
    encryptd.7z
    README.txt