Hi
Do that:
((Shape3D)getChild(TOP)).setAppearance(ap0);
((Shape3D)getChild(LEFT)).setAppearance(ap1);
((Shape3D)getChild(RIGHT)).setAppearance(ap2);
((Shape3D)getChild(FRONT)).setAppearance(ap3);
((Shape3D)getChild(BACK)).setAppearance(ap4);
((Shape3D)getChild(BOTTOM)).setAppearance(ap5);
You should have looked into the source code, it was easy to guess. Java3D is open source, people should look at the documentation and the source code before asking any question here:
https://github.com/hharrison/java3d-utils/blob/master/src/classes/share/com/sun/j3d/utils/geometry/Box.java#L287