You need to add a SelectedItemChanged event to the TreeView control, and in there you can get the selected item and check if it's a child of the Standards item, as follows:
private void ToDoList_SelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
{
TreeViewItem selItem = ((TreeViewItem)ToDoList.SelectedItem);
//If the parent is a treeviewitem, this is a child node.
if (selItem.Parent is TreeViewItem)
{
//Check if the parent treeviewitem is the standards node.
if (((TreeViewItem)selItem.Parent).Header.ToString() == "Standards")
{
//Do stuff here.
}
}
}