As you probably know, the Sorted property of a TreeView applies to all nodes, not just a selected node.
There are two ways I can think (without any testing) of sorting a specific nodes children:
1. Sort them before they are added, if possible
2. Add all the child nodes to an ArrayList and sort the ArrayList. You may need to create a holding class for each child node which implements IComparable if the default sort does not achieve what you want. When the ArrayList is sorted, clear the parent node and re-add the child nodes back in in the sorted order.