<xsl:template match="parent">
       <xsl:apply-templates select="child_a | child_b"/>
       <xsl:apply-templates select="child_c"/>
       <xsl:if test="not(child_c)">
           <child_c/><xsl:comment>added</xsl:comment>
       </xsl:if>
       <xsl:apply-templates select="child_d | child_e"/>
   </xsl:template>

HTH
Anton