I can't seem to find this option either, where is it hidden?

Hello all!

Did you find a solution for the problem? I cannot find the setup.

Thank you.

After looking at the documentation, it appears the defaults have moved. From the docs:

"From Pages & Themes > Page Types, we see a list of all page types currently installed on our site. Next to each, we see a button labeled "Output", among other buttons. This will provide us with a list of compatible Page Templates for this page type. Clicking the "Edit Defaults" button next to one of the available templates, an editable "Default" version of a page template to be used as a master template of sorts for new pages created using that page type and template combination."

I got this but this only relates to new pages. How do you impose the new structure on pages that already exist as per 5.6 vers "Set up on child pages"?

Think I'm missing something?

Anyone get an answer on this? In 5.7 I don't see the "Set up on Child Pages" option anywhere.

April 2016 - still the same problem for me too... what gives?
I've looked everywhere

Found it!

When editing page defaults you need click on the block/stack you want to update globally. The context menu will have "set up on child pages" [see attachment for screenshot]
I knew it had to be something simple

Can't find how to do this with a layout though, just added a block and pushed it to child pages and it didn't appear. Then I rmemebered the block was in the RHS column of a layout but can't see how to push the layout to child pages :o(

Anyone?