In a hierarchical navigation structure such as QuickLaunch, the tree is rooted on a single SPNavigationNode object. This object has a Children property that returns an SPNavigationNodeCollection object with navigation nodes one level down from the root. Each of these nodes has a Children property that can contain a collection of nodes for another level down the hierarchy.
You can get the parent node for a collection by getting the value of the collection's Parent property. You can return to the collection by getting the value of the parent node's Children property.