with Dynamic drop downs on an update page, you need to use the Populate list from array behavior on the body on load tag.
similar to adding the Populate list from array behavior to the parent select list, but instead, use the body tag.
Dynamic Dropdowns also has a behavior for Set Selected in List. this also need to be used to set the parent list and the child list
the behaviors for the body tag should be:
set selected in list - to set the parent item
populate list from array - to popualte the child list
set selected in list - to set the selected item in the child list.