How to compare position of two child of different parent's object.

Hi, I have two object those have different their child objects. I want that my second object's child always come on top as compare with first one, and first object's child place on first in camera, but when second object come then its child always show on top and hide first object's children. I thought that i may add second camera to handle this issue but I am making app of Augmented Reality and following Vuforia sdk so there is only one default ARCamera. Any one experienced this sort of problem kindly help me on this. This will be great for me. Thanks!