Show children of a Taxonomy Term
If I understood you correctly, you want to display nodes that have a term, but not the nodes that have a child term.

You also want to display the child terms.

There are some different ways of handling this, since you are using the default view, you should get a View already that displays the correct nodes - so all you are missing is the terms.

The best way to deal with this, is not through Views, but a module called Taxonomy menu. It will create a menu with the same structure of a vocabulary which can expend, to show child menus. As with menus you get use the block system to add them on the term pages. Beware of a bug with the menu system though see the issue.

A quick fix is to add this in your settings file:

$conf['menu_default_active_menus'] = array(
  'navigation',
  'management',
  'user-menu',
  'main-menu',
  'your-menu-here',
);