Function to get the post count from a given category or taxonomy term or terms, including their children in WordPress.

wp-category-and-children-count.php

<?php
/*
 * Function to get the post count from a given category or taxonomy
 * term or terms, including their children.
 *
 * @param (int|array|string) $term Single integer value, or array of integers or "all"
 * @param (string) $taxonomy
 * @param (array) $args Array of arguments to pass to WP_Query
 * @return $q->found_posts
 *
 */
function ad_get_term_post_count( $term = '', $taxonomy = 'category', $args = [] )
{
    // Lets first validate and sanitize our parameters, on failure, just return false
    if ( !$term ) {
      return false;
    }

    if ( $term !== 'all' ) {
      if ( !is_array($term) ) {
        $term = filter_var( $term, FILTER_VALIDATE_INT );
      } else {
        $term = filter_var_array( $term, FILTER_VALIDATE_INT );
      }
    }

    if ( $taxonomy !== 'category' ) {
      $taxonomy = filter_var( $taxonomy, FILTER_SANITIZE_STRING );
      if ( !taxonomy_exists( $taxonomy ) ) {
        return false;
      }
    }

    if ( $args ) {
      if ( !is_array ) {
        return false;
      }
    }

    // Set our default args
    $defaults = [
      'post_status'    => 'publish',
      'posts_per_page' => 1, // don't need more than 1 to run query
      'fields'         => 'ids'
    ];

    if ( $term !== 'all' ) {
      $defaults['tax_query'] = [
        [
          'taxonomy' => $taxonomy,
          'terms'    => $term
        ]
      ];
    }
    $combined_args = wp_parse_args( $args, $defaults );
    $q = new WP_Query( $combined_args );

    // Return the post count
    return $q->found_posts;
}