Joomla native routing is not quite robust and you can't control how the SEF URL of menu item is built. You can only define menu item alias - a component of URL.

By default aliases of Joomla menu items are relative: aliases of all parent menu items are auto-prepended to SEF URL.

So child menu items have URLs like:

http://site.com/parent-menu-item-alias/child-menu-item-alias

This plugin allows you to switch alias to 'direct' mode and have short URL without aliases of parent menu items:

http://site.com/child-menu-item-alias

Another option is not include alias of certain menu item into SEF URL of child items.