Add new element to xml file in python and preserve indentation

I have two files which look like the following:

parent.xml

<?xml version="1.0" ?>
<persons>
    <person name="Tom" surname="Tomerson"/>
    <person name="Dave" surname="Davidson"/>
</persons>

child.xml

<?xml version="1.0" ?>
<person name="Mike" surname="Michaelson"/>

I want to add the element in the child file to the parent file so that i end up with a parent.xml file that looks like the following:

<?xml version="1.0" ?>
<persons>
    <person name="Tom" surname="Tomerson"/>
    <person name="Dave" surname="Davidson"/>
    <person name="Mike" surname="Michaelson"/>
</persons>

However what i actually end up with is a parent.xml file that looks like :

<?xml version="1.0" ?>
<persons>
    <person name="Tom" surname="Tomerson"/>
    <person name="Dave" surname="Davidson"/>
<person name="Mike" surname="Michaelson"/></persons>

i.e the indentation of the new element does not match the previous elements. And there is no new line inserted after the new element is appended. The code i am trying is the following:

import xml.dom.minidom

class add_xml_fragment:

  def add_fragment(self, file, parentFile):
        childDoc = xml.dom.minidom.parse(file)
        elem = childDoc.documentElement
        parent_doc = xml.dom.minidom.parse(parentFile)
        parent_root = parent_doc.documentElement
        parent_root.appendChild(elem)
        parent_doc.writexml(open(parentFile,'w'))

instance = add_xml_fragment()
instance.add_fragment("child.xml", "parent.xml")

How should i modify the above code to achieve the desired file with correct indentation and newline?