select the body tag in the tag selector.
in the behaviors panel, click the plus button, and select WebAssist -> Dynamic Drop Downs -> Set selected in list. select the parent list, and bid the value to the corresponding column from the recordset
make sure the body unload event is selected.
click the plus button again, and the Populate list from array behavior.
click the plus button again, and select the Set Selected in list behavior again, this time select the child list.