Hi Mel:
Why not specify the value of thisform.cNavWorkarea to the child form's workarea/alias when you activate the child form. Remember that when you do this, the controls will need to be refreshed. In the child form's Activate method have something like :-
thisform.cNavWorkarea = 'ChildWorkArea'
thisform.oStateManager.RefreshAll('Form')
When you re-activate the parent form, you will have to "reverse" this logic.
HTH
-=Gary