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?