Dynamic Dropdowns includes a behavior called Set Selected in List that you will need to use for this.
you will need to add this behavior and the Populate list From Array behavior to the Onload event of the body tag.
Select the body tag, on the behaviors tab, click the plus button. select Webassist -> Dynamic Drop Downs -> Set selected in list.
for the target list, select the parent list, for the value, click the lightning bolt and select the corresponding URL Variable binding.
Now add a populate list from array behavior to populate the child list.
finally add a second set selected in list behavior to set the child list value.