class Model extends \Jenssegers\Mongodb\Model
{
    public function association()
    {
        return $this->embedsMany('ModelChild');
    }
}

class ModelChild extends \Jenssegers\Mongodb\Model
{
}

$model = new Model;
$child = new ChildModel;

$model->association()->associate($child);
$model->save(); // The child's timestamps ARE NOT hydrated

$model = new Model;
$child = new ChildModel;

$model->save();
$model->association()->save($child); // The child's timestamps ARE hydrated