You get the "setup on child pages" option only when you go to the page type defaults.

Go to Dashboard => Pages & Themes => click "Defaults" button from the page type you want to apply the stack to.

There if you now add the stack, after adding that you can click it again and it will give you the "Setup on Child Pages" option.

Please also note that it will be added to any new pages of that page type automatically that you add to the site as it is in the page type defaults.


Best,
Antti

Go to Dashboard => Pages & Themes => click "Defaults" button from the page type you want to apply the stack to.

I can't even find "defaults"

Page Types missing there.

Dashboard => Pages & Themes => PAGE TYPES => click "Defaults".