updating XML attribute value in python

In the below XML, I want to parse it and update the value of "alcohol" to "yes" for all the attributes where age>21. I'm having a problem with it being a node buried inside other nodes. Could someone help me understand how to handle this?

Here's the XML again..

<root xmlns="XYZ" usingPalette="">

<grandParent hostName="XYZ">
<parent>
        <child name="JohnsDad">
            <grandChildren name="John" sex="male" age="22" alcohol="no"/>
        </child>
        <child name="PaulasDad">
            <grandChildren name="Paula" sex="female" age="15" alcoho="no"/>
        </child>
</parent>
</grandParent>     
</root>   

I tried find all and find method using this document here (http://ift.tt/1vO21Oh) but it didn't find it. For example, following code returns no results

for node in tree.findall('.//grandParent'):
    print node