prefix_validate_total_items.php

<?php
/**
 * Custom validation for add-on fields
 */
function prefix_validate_total_child_items( $passed, $product_id, $quantity, $variation_id=null, $cart_item_data=array() ) {

  // The parent product ID that we need to validate
  $parent_product_ID = 9059; // Change this
  if( $parent_product_ID != $product_id ) {
    return $passed;
  }

	// The list of field IDs that need to be checked
  $fields = array( 9061 ); // Change this
  // The combined quantity
  $total = 0;
  // The required quantity
  $required = 15; // Change this

  // Iterate through each field and get combined quantity
	$groups = pewc_get_extra_fields( $product_id );

	if( $groups ) {

		foreach( $groups as $group_id=>$group ) {

			if( isset( $group['items'] ) ) {

				foreach( $group['items'] as $field_id=>$field ) {

					$id = $field['id'];

          if( in_array( $field_id, $fields ) ) {
            // This is one of our fields

            $child_products = ! empty( $field['child_products'] ) ? $field['child_products'] : array();
						foreach( $child_products as $key=>$child_product_id ) {
							if( isset( $_POST[$id . '_child_quantity_' . $child_product_id] ) ) {
								$total += $_POST[$id . '_child_quantity_' . $child_product_id];
							}
						}

          }

        }

      }

    }

  }

	if( $total < $required ) {
		wc_add_notice( 'My validation message', 'error' ); // Change this
		$passed = false;
	}

  return $passed;

}
add_filter( 'woocommerce_add_to_cart_validation', 'prefix_validate_total_child_items', 10, 5 );