Use
<xsl:template match="child">
<tr>
<xsl:if test="position() mod 2 = 1">
<xsl:attribute name="bgcolor">green</xsl:attribute>
</xsl:if>
<td class="normal"><xsl:value-of select="@col1"/></td>
etc
(although I would have used a different CSS class, but that's a CSS question not an XSLT question).
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference