functions.php

<?php

/**
 * Template Hierarchy
 *
 */
function ea_template_hierarchy( $template ) {
	
	if ( is_page() ) {

		$ancestors = get_ancestors( get_the_ID(), 'page' );
		$children = get_pages( array( 'child_of' => get_the_ID() ) );

		// Product page has an ID of 25
		if ( in_array( 25, $ancestors ) || 25 == get_the_ID() ) {

			if ( !empty( $children ) ) {
				$template = get_query_template( 'product-listing' );

			} else { 
				$template = get_query_template( 'product' );
			}

		}

	}
		
	return $template;
}
add_filter( 'template_include', 'ea_template_hierarchy' );