def _get_all_children_id(base, tag):
    """Get a list of ids of all child elements with a given tag.

    Args:
        base: The base node element. (lxml node)
        tag: Child elements group tag used to select the elements. (string)

    Returns:
        A list with all child id strings found or an empty list.
    """
    ret = []
    for node in base:
        if node.tag.replace('{http://www.rdml.org}', '') == tag:
            ret.append(node.get('id'))
    return ret
