I think you cannot achieve this.
The following is from the remark section of the
SetWindowPos[
^]:
A non-topmost window can own a topmost window, but the reverse cannot occur. Any window (for example, a dialog box) owned by a topmost window is itself made a topmost window, to ensure that all owned windows stay above their owner.
As your pop up is owned by the child dialog it cannot be hidden behind it.