INSERT INTO "NODES" (ID, NAME) VALUES (1, 'test parent');
INSERT INTO "NODES" (ID, PARENTID, NAME) VALUES (2,1,'test child 1');
INSERT INTO "NODES" (ID, PARENTID, NAME) VALUES (3,1,'test child 2');
INSERT INTO "NODES" (ID, PARENTID, NAME) VALUES (3,2,'test child 3');

<nstst:nodes xmlns:ns="test" xmlns:ns1="test1" xmlns:ns2="test2">
    <ns1:node>
        <ns2:name>test parent</name>
        <!-- extra info -->
        <nodes>
            <node>
                <name>test child 1</name>
                <!-- extra info and child nodes -->
            </node>
            <node>
                <name>test child 2</name>
                <!-- extra info and child nodes -->
            </node>
        </nodes>
    </node>
</nodes>

SELECT DBMS_XMLGEN.getXMLType(
     DBMS_XMLGEN.newContextFromHierarchy('
        SELECT LEVEL,
           case
             when CONNECT_BY_ISLEAF = 1 then
              XMLELEMENT("nodes", XMLELEMENT("ns1:node",
                    xmlattributes(''test'' as "xmlns:ns",
                                ''test1'' as "xmlns:ns1",
                                ''test2'' as "xmlns:ns2"),
                xmlelement("ns2:name", n.code)))
             when CONNECT_BY_ISLEAF = 0 then
              XMLELEMENT("nodes", XMLELEMENT("ns1:node",
                    xmlattributes(''test'' as "xmlns:ns",
                                ''test1'' as "xmlns:ns1",
                                ''test2'' as "xmlns:ns2"),
                xmlelement("ns2:name", n.code)), null)
           end
          FROM NODES n
         START WITH n.CODE = ''JUST''
        CONNECT BY PRIOR n.ID = n.PARENTID '
        )
       ) FROM DUAL;

<?xml version="1.0"?>
<nodes>
  <ns1:node xmlns:ns="test" xmlns:ns1="test1" xmlns:ns2="test2">
    <ns2:name>JUST</ns2:name>
  </ns1:node>
  <nodes>
    <ns1:node xmlns:ns="test" xmlns:ns1="test1" xmlns:ns2="test2">
      <ns2:name>test child 1</ns2:name>
    </ns1:node>
    <nodes>
      <ns1:node xmlns:ns="test" xmlns:ns1="test1" xmlns:ns2="test2">
        <ns2:name>test child 3</ns2:name>
      </ns1:node>
    </nodes>
  </nodes>
  <nodes>
    <ns1:node xmlns:ns="test" xmlns:ns1="test1" xmlns:ns2="test2">
      <ns2:name>test child 2</ns2:name>
    </ns1:node>
  </nodes>
</nodes>