An example child item would just fall under the parent item in the path hierarchy, no need for plid.
e.g
$items['admin/sponsorship/child'] = array(
'title' => 'Sponsor child',
'description' => 'This is where all of the child sponsorship data is stored',
'page callback' => 'mymodule_sponsor_child_page',
'access arguments' => array('access sponsorship section'),
'type' => MENU_NORMAL_ITEM
);
system_menu has some good examples of menu item containers, using system_admin_menu_block_page, e.g admin/appearance/*
Make sure you clear menu cache to get any new menu item to show up there.