I did some more research and found the equivalent method for adding child TreeViewItems
to parent TreeViewItems
in WPF.
This is the change I made to my code:
//ADD T_ITEM TO PARENT TREEVIEW
private void button1_Click(object sender, RoutedEventArgs e)
{
TreeViewItem item = new TreeViewItem();
item.Header = textBox1.Text;
Parent.Items.Add(item);
}