fire two related events at once (child event needs its parent to fire first )

HI
I have two DropDownLists , (parent and child) , when I change the parent index, child should chnage its value accordinglly and do something else. this is working fine when I manualy change parrent DDL by mouse (in run time).
Now the part that I can not sort out begins : If I change the value of parent DDL through coding (when you press a button data will retreive from database and change Parent DDL values) the child wont fire . (I think I want to fire two related events in row at one shot(load). ) is there any way to do that ?
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