Check occurence of two consecutive XML tag in Unix Shell

I have the following XML File:

<PARENT_TAG>
    <CHILD_TAG>
        <DETAIL_1_TAG></DETAIL_1_TAG>
        <DETAIL_2_TAG></DETAIL_2_TAG>
        <DETAIL_3_TAG></DETAIL_3_TAG>
        <DETAIL_4_TAG></DETAIL_4_TAG>
    </CHILD_TAG>
</PARENT_TAG>

which can be formatted even as

<PARENT_TAG><CHILD_TAG><DETAIL_1_TAG> ...  </CHILD_TAG></PARENT_TAG>

I would like to extract the name of the xml tag following using grep,awk,sed. In this case, I would extract "CHILD_TAG".