How to replace function in child theme?

Question

There is a function in the parent theme:

function latest_comments($list_number = 5, $cut_length = 50)
{
    global $wpdb, $output;
    $comments = $wpdb->get_results($wpdb->prepare("SELECT comment_ID, comment_post_ID, comment_author, comment_author_email, comment_date, comment_content FROM {$wpdb->comments} LEFT OUTER JOIN {$wpdb->posts} ON {$wpdb->comments}.comment_post_ID = {$wpdb->posts}.ID WHERE comment_approved = '1' AND (comment_type = '' OR comment_type = 'comment') AND user_id != '1' AND post_password = '' ORDER BY comment_date_gmt DESC LIMIT %d", $list_number));
    foreach ($comments as $comment) {
        $nickname = esc_attr($comment->comment_author) ?: __('匿名', 'kratos');
        $output .= '<a href="' . get_the_permalink($comment->comment_post_ID) . '#commentform">
            <div class="meta clearfix">
                <div class="avatar float-left">' . get_avatar($comment, 60) . '</div>
                <div class="profile d-block">
                    <span class="date">' . $nickname . ' ' . __('发布于 ', 'kratos') . timeago($comment->comment_date) . '（' . wp_date(__('m月d日', 'kratos'), strtotime($comment->comment_date)) . '）</span>
                    <span class="message d-block">' . convert_smilies(esc_attr(string_cut(strip_tags($comment->comment_content), $cut_length))) . '</span>
                </div>
            </div>
        </a>';
    }
    return $output;
}

And I want to replace it with following function in child theme(the difference is elimination of avatar float-left):

function latest_comments($list_number = 5, $cut_length = 50)
{
    global $wpdb, $output;
    $comments = $wpdb->get_results($wpdb->prepare("SELECT comment_ID, comment_post_ID, comment_author, comment_author_email, comment_date, comment_content FROM {$wpdb->comments} LEFT OUTER JOIN {$wpdb->posts} ON {$wpdb->comments}.comment_post_ID = {$wpdb->posts}.ID WHERE comment_approved = '1' AND (comment_type = '' OR comment_type = 'comment') AND user_id != '1' AND post_password = '' ORDER BY comment_date_gmt DESC LIMIT %d", $list_number));
    foreach ($comments as $comment) {
        $nickname = esc_attr($comment->comment_author) ?: __('匿名', 'kratos');
        $output .= '<a href="' . get_the_permalink($comment->comment_post_ID) . '#commentform">
            <div class="meta clearfix">
                <div class="profile d-block">
                    <span class="date">' . $nickname . ' ' . __('发布于 ', 'kratos') . timeago($comment->comment_date) . '（' . wp_date(__('m月d日', 'kratos'), strtotime($comment->comment_date)) . '）</span>
                    <span class="message d-block">' . convert_smilies(esc_attr(string_cut(strip_tags($comment->comment_content), $cut_length))) . '</span>
                </div>
            </div>
        </a>';
    }
    return $output;
}

I have search two methods: the first is pluggable function, which is not fit to me as the parent theme is the third-party. The second is specify the priority of the function. However there is no add_action code and then I can't insert add_action('after_setup_theme', 'my_child_theme_function', 20) as the tutorial do. Can anyone give other methods?