A child page has a parent, this makes the URL path show both parent and child page string on the URL, thats great for SEO!
Navigate to the page you want to edit and hover over the item, then select Quick Edit.
Click the Parent dropdown and select the page you want to make as its parent.
EG: if we select the About Us page as the parent for the Contact Us page the page URL will change to www.yoursite.com/about-us/contact-us