from lxml import etree as ET

parser = ET.XMLParser(remove_blank_text=True)
tree = ET.parse(file_xml,parser)
root = tree.getroot()

child1 = root.find('.//{http://www.isotc211.org/2005/gmd}child1')
parent = child1.getparent()
id = root.index(child1)

child2 = ET.Element('{http://www.isotc211.org/2005/gmd}child2', nsmap = {'srv':'http://www.isotc211.org/2005/srv'})
child_child2 = ET.SubElement(child2,'{http://www.isotc211.org/2005/gco}CharacterString')
child_child2.text = 'text'

parent.insert(id+1,child2)

ET.ElementTree(root).write(file_new_xml, pretty_print = True)