Apply default ordering of child pages by menu_order, title

Querying child posts does not currently apply any ordering. Perhaps this should be parameterized, but sorting on "menu_order title" is a safe default. [WordPress itself uses this as default to sort hierarchical post listings](https://github.com/WordPress/WordPress/blob/04cb00c9e12bb7e325f76091763986eeac7dd804/wp-admin/includes/post.php#L875).