XElement => Add children nodes at run time

Try this:

var x = new XElement("root",
             new XElement("name", "AAA"),
             new XElement("last", "BBB"),
             new XElement("children",
                 from c in family
                 select new XElement("child",
                             new XElement("name", "XXX"),
                             new XElement("last", "TTT")
                        )
             )
        );