Perhaps the OnEnter event of the appropriate subform control can update that for you? You could use something like:
[Forms]![frmMyMainForm].[MyYesNo]=True
as the underlying code for this event which should check the box for you when the subform is entered (gets focus; but not to be confused with the OnFocus event). Then if you wanted to return the checkbox to false use the same line in the OnExit event of the same subform and change "True" to "False".