Generally speaking, you cannot change a plugin through a child theme, unless you use hooks the author has provided, or unless the author has allowed some of the plugin files to be overwritten.
However, as you only wish to change translatable text, you don’t need any of this.
- Install the plugin Loco Translate. Don’t be scared, you won’t
actually be translating anything to another language – simply
overriding the default translation files for the plugin. - Activate Loco Translate and go to Loco Translate->Plugins.
- You will be presented with a list of plugins. Find the plugin that contains the text, click on it and then pick “Create Template”.
- Once it has been created go to “New Language” and create a template
for the language you are using – if it’s the default WP setup, this
should be English(United States). - You will be redirected to the translation editor. There you will see
all the plugin strings. Find the string you wish to change, change it
(by adding a new text in the bottom box) and press “Save”.
The above is a bit of a summary of how the plugin works. For more info about how to use Loco Translate: https://localise.biz/wordpress/plugin/manual/editor