<?xml version="1.0" encoding="utf-8" ?> 
<Root>
  <ParentNode id="1"><ChildNode/></ParentNode>
  <ParentNode id="2"><ChildNode printParentNode="yes"/></ParentNode>
  <ParentNode id="3"><ChildNode printParentNode="no"/></ParentNode>
  <ParentNode id="4"/>
</Root>

<?xml version="1.0" encoding="UTF-8" ?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:output method="xml" omit-xml-declaration="no" encoding="utf-8"/>

  <xsl:template match="/">
    <Root>
      <xsl:for-each select="/Root/ParentNode">
        <xsl:if test="count(ChildNode) &lt; 1 or ChildNode/@printParentNode = 'yes'">
          <ParentNode id="{@id}"><xsl:apply-templates/></ParentNode>
        </xsl:if>
      </xsl:for-each>
    </Root>
  </xsl:template>
  
</xsl:stylesheet>