Plugins are not kept in the theme directory. It doesn’t matter if you are using a child theme or not. Maybe you’re asking a different question? Are you asking how to make a “child-plugin”?
Plugins are generally kept here: wp-content/plugins.
Whereas child themes and themes are kept here: wp-content/themes
As far as “child-plugins” there is not an official way to do it, but I think this article sums up pretty nicely what you could do to accomplish that, https://iandunn.name/the-right-way-to-customize-a-wordpress-plugin/ .