Option to apply page rules that display messages to child pages

If I have a page rule that shows a message, it would be very useful if there was an option with the rule to apply it to all child pages of the page that has the rule.

E.g., if I have a page that shows the user a warning (e.g., because they need to provide some extra information) I'd like that warning to show up on the child pages as well, because it's very likely that those child pages will use that extra information in some way.