Access only specific folders via SFTP

I have a VPS running Ubuntu Server 14.04.

Here's a selection of my directory structure:

/
    home/
        my_user/
    var/
        www/
            wp-content/
                themes/
                    my-theme/
                    my-theme-child/

Permissions for the directories are 755 and files are 644.

Say I hire a developer, my_user, to work on a theme for me. I want them to be able to do anything they want to their home folder. I also want them to be able to read my-theme and read and write my-theme-child, but nothing more. They shouldn't even be able to see the directory structure of any other part of the server.

I've done some reading and I think a chroot jail would work nicely to keep my developer in their home directory, but I can't figure out how to give them access to my-theme and my-theme-child in addition to their current access.

How can I achieve this?