When using wp_list_pages() to display the page hierarchy for navigation purposes, it would be useful to be able to modify the css placed into the output. For example, I want to designate which pages have children by styling a right-hand arrow only on those pages. This requires a class (e.g., "parent" or "ancestor") to differentiate from Pages that have no children.
This is already done in a way for the current_page (current_page_ancestor, current_page_parent). And we can obviously figure out child pages with CSS (ul li ul li). But going the other direction is difficult.
Instead of requesting a specific code change related to that case, perhaps a new filter hook placed in the middle of the start_el() function would deal with other cases.
If there was a filter hook somewhere between lines 1175 and 1193 in classes.php that examined the args and css_class arrays, then I could check the state of a page at that point (e.g.,
$args['has_children']) and modify the css_class array appropriately.