How do i add children pages to all pages everywhere?

[Robert Yoon]

Since they got rid of documentation theme,

users are complaining this feature doesnt exist.

How can I add a footer to every page to include this like in confluence 5

[Nic Brough -Adaptavist-]

Ah, that's not "add child pages" really.  I was going to question why you'd want to add a child page everywhere, but then say that I think Script Runner for Confluence can do it (if you don't want to write REST scripts yourself)

However, the footer is a different story.  You should be able to amend the global html footer to get something like a footer, but, as you probably already suspect, your best option may well be a custom theme that supports custom footers.  Have a look in the marketplace for one-off themes, or Brikit and Refined if you want lots of easy flexibility, and Theme Builder if you want a huge amount of theming power (but, be warned, you do need to commit to that one, it's not "install, do few minutes easy config and move on")