If you have activated the plugin, access a confluence page that has one or more child pages.
Now you can access the “Delete page tree” function via the “Tools” menu. This takes you to a short overview. That lists which pages are going to be deleted. In addition to this it shows pages that are not going to be deleted because of set permissions.
After verifying that this is what you want to delete, press the delete button on the bottom of the page.
You can get the plugin at https://marketplace.atlassian.com/plugins/at.rumpelcoder.confluence.deletepagetree