Ok, I'm no guru, but here's my opinions none-the-less:
If you are designing a scenegraph, then most probably you will be concerned about culling large parts of the graph when possible, eg: the traditional quad-tree terrain where if your camera is facing east, and you are east of the midpoint, you can instantly discard 2 of the 4 initial child nodes. So, you are going depth-first, and testing whether to traverse further at each node.
You're always moving down the tree, never going up (until you exhaust the children of the current node), so by virtue of this fact you will always have access to the "parent" node. Therefore you only need one-way references to the child nodes.