in the tag selector, select the body tag.
on the Behaviors panel, click the plus button and select WebAssist -> WA Dynamic DropDowns -> Set Selected In List.
For the target list, select the parent list.
For the value, click the lightning bolt icon and select the corresponding element from the Validated Entries Collection.
click the plus button again, this time select WebAssist -> WA Dynamic DropDowns -> Populate List from Array. and set up the dynamic array the same as you did for the onchange event of the parent select list.
click the plus button once more and select WebAssist -> WA Dynamic DropDowns -> Set Selected In List.
For the target list, select the child list.
For the value, click the lightning bolt icon and select the corresponding element from the Validated Entries Collection.
make sure these 3 behaviors are added to the onload event of the body tag.