Permissions – Prevent Deletion of Certain Files in User-Owned Directory

Directory permissions:

• The write bit allows the affected user to create, rename, or delete files within the directory, and modify the directory's attributes
• The read bit allows the affected user to list the files within the directory
• The execute bit allows the affected user to enter the directory, and access files and directories inside
• The sticky bit states that files and directories within that directory may only be deleted or renamed by their owner (or root)
  

You can save the files under the ownership of root user and thus this will require them to use password before accessing those files.
As said in directory permissions, you can take away 'write bit' and 'execute bit' thus not allowing them to enter directory. only give them read permission so that they can view files without altering and deleting them.
you can learn the use of sticky bit (link here) and disabling alter and delete feature on every file inside that directory

If they have root password then hiding files is only the way to protect your files and root is god of the system, if they have root password, so they are the real god of your system !

It's wrong. To delete something you need write permissions on the directory containing it. That also goes for directories themselves: to delete a directory, you need (at a minimum) write permissions on the parent directory. You may need write permissions on the directory as well, but that's not enough by itself.

Write permissions on the directory itself are needed when the directory is not empty. In that case, you need to clear out the directory first by deleting everything in it, so you need write permissions on all sub-directories as well (recursively). Then you can delete the directory itself if you have write permissions on the parent.