The current behavior of Confluence is to hide both child pages list in the lower part of the page and child page tree in the side navigation bar of "Documentation Theme".
This behavior require user to click at least once to see the list of child pages of the parent page accessed and due to the design of the new drop down button it is hard to differentiate pages that have children and the one that doesn't have child pages. Since opening up the whole child page list in the lower part of the page could be aesthetically unpleasing if the number of child pages is huge, we should provide the option for this at side bar navigation.
Since "Documentation Theme" provide side navigation bar to, it would be great if administrator or users of Confluence has the ability to configure it to open the child page tree automatically when the parent page accessed.
Hence user will be able to use side navigation bar to see the list of child pages without clicking it.
For administrator, this configuration should be provided at theme configuration. For users, this configuration should be provided at user profile preferences option.
Another method is to improve the button icon to be a bigger and higher contrast so user would at least able to differentiate it easily.