PHP

<div class="side_archive_menu">
    <?php
    $year_prev = null;
    $months = $wpdb->get_results("SELECT DISTINCT MONTH( post_date ) AS month ,
                                      YEAR( post_date ) AS year,
                                      COUNT( id ) as post_count FROM $wpdb->posts
                                      WHERE post_status = 'publish' and post_date <= now( )
                                      and post_type = 'post'
                                      GROUP BY month , year
                                      ORDER BY post_date DESC");
    foreach($months as $month) :
    $year_current = $month->year;
    if ($year_current != $year_prev){
    if ($year_prev != null){?>
        <?php } ?>
  <p class="toggle_btn"><?php echo $month->year; ?>年</p>
  <ul class="child">
    <?php } ?>
    <li>
        <a href="<?php bloginfo('url') ?>/<?php echo $month->year; ?>/<?php echo date("m", mktime(0, 0, 0, $month->month, 1, $month->year)) ?>">
            <?php echo date("n", mktime(0, 0, 0, $month->month, 1, $month->year)) ?>月
            (<?php echo $month->post_count; ?>)
        </a>
    </li>
    <?php $year_prev = $year_current;
    endforeach; ?>
  </ul>
</div>
<!-- /.side_archive_menu -->

抽出されるHTML

<div class="side_archive_menu">
	<p class="toggle_btn">2018年</p>
	<ul class="child">
		<li><a href="">11月(1)</a></li>
		<li><a href="">10月(14)</a></li>
		<li><a href="">9月(1)</a></li>
	</ul>
</div>
<!-- /.side_archive_menu -->

CSS

.child {
  display: none;
}

jQuery

$(".toggle_btn").on("click", function() {
     $(this).next().slideToggle();
     $(this).toggleClass("active");
 });