Child theme is optional. It includes minim files needed to setup a child theme for clients who wish to change theme default code /css (style.css)
Install wprentals.zip from Themes – Appearance – Upload – help http://help.wprentals.org/2015/10/28/theme-installation
Then install wprentals-child.zip the same way. Activate child theme.
Please see video tutorial
Save Primary Menu and Footer Menu from Admin – Menus http://help.wprentals.org/2015/10/28/footer-menu/
You can start adding CSS modifications right away.
For more complex changes documentation, go to http://help.wpresidence.net/2015/12/11/technical-how-to-child-theme/
Changes in child theme are not affected during a theme update. You may still need to review these changes in case new theme updates are done in the same files modified in child theme. If this is the case you must adapt your child theme to new theme updates.