instead of using the set value equal to option, you will need to apply the following Dynamic Drop Down Behaviors to the onLoad event of the body tag:
1) Set Selected In List - Target List: Parent List, Value: Use the binding from the recordset for the parent list
2) Populate List From Array - Set this up the same way you would for the Parent list form element on change event.
3) Set Selected in list - Target List: Child List List, Value: Use the binding from the recordset for the child list