get_cat_count_with_child.php

<?php

//Get post count of a category including sub-categories in WordPress
function wp_get_cat_postcount( $id ) {
    $cat      = get_category( $id );
    $count    = (int) $cat->count;
    $taxonomy = 'category';
    $args     = array(
        'child_of' => $id,
    );
    $tax_terms = get_terms( $taxonomy, $args );
    foreach ( $tax_terms as $tax_term ) {
        $count += $tax_term->count;
    }

    return $count;
}