It depend if you're using billboards or not.
Generally speaking, in 3D the sorting is made by the depth position difference from the camera.
Assuming you're not using billboard, you just have to change by a very small number the Z position of the child sprite which should appear on top. If you want the same sprite in both direction, you will need to handle it with "Cull Mode" (Geometry->Material Override->Parameters) it make the sprite vissible or not depending the facing (from the camera).