Did you happen to notice the drop down indicators on the main menu above? Those drop down indicators are created simply by adding the following to your style.css:
.genesis-nav-menu > .menu-item-has-children > a:after { content: "\f140"; font: normal 16px/1 'dashicons'; display: inline-block; -webkit-font-smoothing: antialiased; padding-left: 3px; vertical-align: top; }
The css takes advantage of WordPress's built-in dashicons. If you want a drop down indicator that looks different from what this site uses, you could change the content line to content: “\f347"; for an alternate down arrow () or a standard down arrow () with content: “\f346";.
If you want to include drop down indicators on sub-menus, add .genesis-nav-menu .sub-menu > .menu-item-has-children > a:after to your block title. You'll still have to fiddle with menu item spacing and menu width, but I'll leave that to you.