functions.php

<?php
/**
 * Yoast SEO breadcrumbs - link directly to child page when the parent has our
 * "first child redirect" template.
 *
 * @param array $crumbs The complete list of breadcrumbs.
 *
 * @return array
 */
function kanuka_wpseo_breadcrumbs_first_child_redirect( $crumbs ) {
	foreach ( $crumbs as $key => $link ) {

		// Bail if we don't have a post ID.
		if ( ! isset( $link['id'] ) || empty( $link['id'] ) ) {
			continue;
		}

		$template = get_page_template_slug( $link['id'] );

		// Bail if the post doesn't have a template.
		if ( empty( $template ) ) {
			continue;
		}

		// Bail if this isn't our first child redirect template.
		if ( 'page-templates/first-child-redirect.php' !== $template ) {
			continue;
		}

		// Grab the first child.
		$args = array(
			'numberposts' => 1,
			'orderby'     => 'menu_order',
			'order'       => 'ASC',
			'post_type'   => 'page',
			'post_status' => 'publish',
			'post_parent' => $link['id'],
		);
		$first_child = get_children( $args );

		// Replace breadcrumb link with first child URL.
		$crumbs[ $key ]['url'] = get_permalink( array_pop( $first_child )->ID );
	}

	return $crumbs;
}
add_filter( 'wpseo_breadcrumb_links', 'kanuka_wpseo_breadcrumbs_first_child_redirect' );