Forcing iFrame content displayed on its wordpress parent page

I am building a Wordpress multisite.

I have iframed several of my subsites. (You can have a look at

Now I want people directly accessing to my subsites through google or other search engines automatically redirected to the parent site where the corresponding subsites are shown in an iframe.

There is a wordpress plugin called "force-frame," which "allows you to force your WordPress website inside an iframe in another website, with cross-domain support."

The plugin is supposed to do what I desire if it works. Nevertheless, i don't understand the plugin's installation instruction, which says "Add the following snippet inside the HTML of the parent."

HTML of the parent? I have no clue as to the html of the parent.

Could you explain on how to add a snippet inside the HTML of the parent and share your insight on how to make iframed subsites always displayed within the parent page?