ErsatzTV is not involved with Plex's Parental Controls at all, and according to that page, Plex's Parental Controls do not apply to Live TV (which is what ErsatzTV is).
Plex parents can create managed user accounts for their children and apply a preset profile to it. These profiles are designed to restrict access to content based on content ratings. These affect media accessed from “movie” and “TV” libraries on a Plex Media Server as well as Video on Demand content. (It does not apply to Live TV content or channels)
Generally speaking, ErsatzTV channels are "all or nothing" - there is one list of all channels. You could perhaps investigate different clients that respect the rating metadata that ErsatzTV includes in the XMLTV: if you see ratings when browsing your content in ETV, those ratings will be included in the generated XMLTV.