How do you insert a child node in a TreeView Control in WPF C#?

How do you insert a child node in a TreeView Control in WPF C#?
I have a TreeView control that I have created in XAML in a WPF program using C#.
After adding a couple nodes at the root level, I have written code that loops through the tree structure like so:
            ItemCollection items = treeView1.Items;
            foreach (TreeViewItem n in items)
            {
              ...
            }
Once I find the place in this loop where I want to include a child node, how do I go about inserting a child?