Return a sorted list of all the children items of 'item'.

def get_item_children(item):
    """Return a sorted list of all the children items of 'item'."""
    children = [item.child(index) for index in range(item.childCount())]
    for child in children[:]:
        others = get_item_children(child)
        if others is not None:
            children += others
    return sorted(children, key=lambda child: child.line)