If you need any changes done to a theme that you did not author, a child theme is a must (if the changes involves direct changes to the functionality and look of the theme itself), otherwise, if you need changes to the site, you would rather use a plugin instead of child theme. In such a cse, a child theme is not needed.
It really does not matter one way or the other, it all boils down to that you should never make changes to a theme or plugin you did not author, this goes for core files as well, always use a plugin and/or a child theme to apply changes or to add or remove features.
If you do not need any changes, it is obvious that you do not need a custom plugin or child theme