Pages-filed also filtering subpages

I have a normal pages-field an set a query to filter the page-picker:

        label: Related Pages
        type: pages
        query: site.pages.filterBy('template', 'default').listed

Is it possible to filter also the child-pages if the user clicked on the > to see the children

No, that’s not possible, at least not out of the box. You would need a custom field.

Or you filter everything, but then you end up with a flat list of options-