We have namespaces with subpages, it should be pretty natural to add a simple way to look for subpages of pages in those namespaces from the toolbox.

There have been questions about this on Stackoverflow before, people have made scripts for this from 2007. I suggest that this would be more useful than stuff like Special:RelatedChangesLinked for people who edit templates and modules, for example, and there is little to none bloating in having this on MediaWiki level (since this would concern pages outside of main space in most projects).

This should act like Special:PrefixIndex/{{FULLPAGENAME}} and be called View subpages. There needs to be a discussion whether to have this link displayed on subpages themselves, but even without going into that I would guess that it would be an improvement.