def get_child_values(parent, names):
        """ return a list of values for the specified child fields. If field not in Element then replace with nan. """
        vals = []
        for name in names:
            if parent.hasElement(name):
                vals.append(XmlHelper.as_value(parent.getElement(name)))
            else:
                vals.append(np.nan)
        return vals
return a list of values for the specified child fields. If field not in Element then replace with nan.