Filter validation message for maximum number of child products

pewc_filter_max_children_validation_notice.php

<?php
/**
 * Filter validation message for maximum number of child products
 */
function prefix_filter_max_children_validation_notice( $message, $field_label, $max_products ) {
  // You can write a custom message here
  // $field_label is the name of the field
  // $max_products is the maximum number of products allowed 
  return $message;
}
add_filter( 'pewc_filter_max_children_validation_notice', 'prefix_filter_max_children_validation_notice', 10, 3 );