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?

Have a look at,

xaml markup

            <TreeView Name="tree1">
                <TreeViewItem Header="One">
                <TreeViewItem Header="Two">


Dictionary<string, List<string>> items = new Dictionary<string, List<string>>()
                {"One",new List<string>() {"A","B","C"}},
                {"Two",new List<string>() {"P","Q"}},

            foreach (TreeViewItem tv in tree1.Items)
                foreach (string item in items[tv.Header.ToString()])