OK if I understand you it is populating the parent select list, but not the child... correct?
Try updating:
<body onload="WA_setSelectionsInChildList(MM_findObj('Pr oductCategoryID'),<?php echo $row_WADAProducts['ProductCategoryID']; ?>)">
and make it:
<body onload="WA_setSelectionsInChildList(MM_findObj('Pr oductCategoryID'),'<?php echo $row_WADAProducts['ProductCategoryID']; ?>')">