$subnodes = [];

$tree = Drupal::menuTree()->load('internal', new DrupalCoreMenuMenuTreeParameters());
foreach ($tree as $item) {
    if ($item->link->getRouteParameters()['node'] == $node) {
        foreach($item->subtree as $subitem) {
            $subnodes[] = $subitem->link->getRouteParameters()['node'];
        }
    }
}

var_dump($subnodes); // array of ids