One of the behaviors that Dynamic DropDowns includes is the set Selected in list behavior.
To control the initial loading state of a page, you can add behaviors to the Onload event of the body tag.
Add the Set Selected in list behavior to the onload event to set the parent list, then add a Populate List From Array behavior to the onload event to populate the child list and end with a set selected in list to set the selected item in the child list.