I understand what kind of functinality you want to achieve. You can do the collapsing of only one node of your navigation tree by allowing collapsing when editing a navigation widget and you will get this result ( see tree_navigation_collapsing.png). The problem with that is when you select a child page the navigation looses its current state and becomes non expanded. You need to save the state state of that navigation in session for example because when you are clicking on child links they cause postback and the control state is lost.
You may find it useful to consider using RAD Tree View control which comes with the Sitefinity installation. An online demo is available here
which will demonstrate its capabilities which in my opinion you are trying to achieve. Here is a link
to the TreeView control documentation. Also this
topic in the documentation will help you disables PostBacks because the item navigate to the particular URL rather than posting back and loose state.
the Telerik team