primary-cat-part-1.php

<?php
public function __construct() {
		add_action( 'rest_api_init', function() {
			register_rest_route( SITE_PREFIX . '/v1', '/yoastprimary/(?P<id>\d+)', array(
				'methods' => 'GET',
				'callback' => array( $this, 'get_posts_by_primary_cat' ),
			) );
		} );
	}

primary-cat-part-2.php

<?php
public function get_posts_by_primary_cat( $data ) {
		$child_cats = get_categories( array( 'parent' => $data['id'] ) );
		$children = array();
		foreach( $child_cats as $child ) {
			$children[] = (String)$child->term_id;
		}
		$children[] = $data['id'];

		$posts = new WP_Query(
			array(
				'post_status' => 'publish',
				'meta_query' => array(
					array(
						'key' => '_yoast_wpseo_primary_category',
						'value' => $children,
						'compare' => 'IN',
					),
				)
			)
		);
		$return = array();
		while( $posts->have_posts() ) {
			$posts->the_post();
			$return_post = array();
			if ( ! get_the_post_thumbnail_url() ) {
        // I set this constant in my theme's function.php
				$return_post['thumb'] = CHILD_THEME_URI . '/assets/imgs/default-thumbnail.jpg';
			} else {
				$return_post['thumb'] = get_the_post_thumbnail_url();
			}
			$return_post['link'] = get_the_permalink();
			$return_post['title'] = get_the_title();
			$return_post['time'] = get_the_time( 'F d, Y' );
			$return[] = $return_post;
		}
		return $return;
	}

primary-cat.php

<?php

class Rest_Api_Posts_By_Primary_Category {
	public function __construct() {
		add_action( 'rest_api_init', function() {
			register_rest_route( SITE_PREFIX . '/v1', '/yoastprimary/(?P<id>\d+)', array(
				'methods' => 'GET',
				'callback' => array( $this, 'get_posts_by_primary_cat' ),
			) );
		} );
	}

	public function get_posts_by_primary_cat( $data ) {
		$child_cats = get_categories( array( 'parent' => $data['id'] ) );
		$children = array();
		foreach( $child_cats as $child ) {
			$children[] = (String)$child->term_id;
		}
		$children[] = $data['id'];

		$posts = new WP_Query(
			array(
				'post_status' => 'publish',
				'meta_query' => array(
					array(
						'key' => '_yoast_wpseo_primary_category',
						'value' => $children,
						'compare' => 'IN',
					),
				)
			)
		);
		$return = array();
		while( $posts->have_posts() ) {
			$posts->the_post();
			$return_post = array();
			if ( ! get_the_post_thumbnail_url() ) {
        // I set this constant in my theme's function.php
				$return_post['thumb'] = CHILD_THEME_URI . '/assets/imgs/default-thumbnail.jpg';
			} else {
				$return_post['thumb'] = get_the_post_thumbnail_url();
			}
			$return_post['link'] = get_the_permalink();
			$return_post['title'] = get_the_title();
			$return_post['time'] = get_the_time( 'F d, Y' );
			$return[] = $return_post;
		}
		return $return;
	}
}