You can check the box Show subsites in the Subsites and Pages section of the Site Navigation Settings for a site. By doing so, each added subsite beneath that site will be added automatically to the Quick Launch navigation element. However, this is limited to 50 subsites. Once you are adding the 51th subsite, it will not be added anymore automatically to the Quick Launch. This can be pretty confusing for your end-users. Especially if you are having fly-out menus in the Global Navigation element on top op the page (there the 51th or above subsite won't appear either).