Render a view with a child view?

I am trying to send an email with a specific "layout" (for header/footer) with a specific template (main view of the email, e.g. 2-column layout etc.).

The $mainView html has: <?= $this->content ?> but this is always NULL when it should be the $childView html with all variables.

Here is what I have in a MailService.php file:

public function createMessage($to, $subject, $template, $variables) : Message
{
    // Create mail
    $message = new Message();

    // Create mail body
    $mainView   = new ViewModel();
    $mainView->setTerminal(true);
    $mainView->setTemplate('layout/mail.phtml');

    $childView  = new ViewModel();
    $childView->setTerminal(true);
    $childView->setTemplate($template);
    $childView->setVariables($variables);

    $mainView->addChild($childView, 'content');

    $msgRender  = $this->viewRenderer->render($mainView);

    $body = new Part($msgRender);
    $body->setType(Mime::TYPE_HTML);

    $messageBody = new MessageBody();
    $messageBody->addPart($body);
    $message->setBody($messageBody);

    $message->setTo($to);
    $message->setSubject($subject);

    return $message;
}

$this->viewRenderer is injected in the Factory:

/** @var RendererInterface $viewRenderer */
$viewRenderer = $serviceLocator->get('viewrenderer');

Ok I got it working ! :) I had to render the childView then set this render as the view variable for the mainView

public function createMessage($to, $subject, $template, $variables) : Message
{
    // Create mail
    $message = new Message();

    // Create mail body
    $mainView   = new ViewModel();
    $mainView->setTerminal(true);
    $mainView->setTemplate('layout/mail.phtml');

    $childView  = new ViewModel();
    $childView->setTemplate($template);
    $childView->setVariables($variables);
    $childRender = $this->viewRenderer->render($childView); // render child

    $mainView->setVariable('content', $childRender); // set childRender to mainView

    $msgRender  = $this->viewRenderer->render($mainView);

    $body = new Part($msgRender);
    $body->setType(Mime::TYPE_HTML);

    $messageBody = new MessageBody();
    $messageBody->addPart($body);
    $message->setBody($messageBody);

    $message->setTo($to);
    $message->setSubject($subject);

    return $message;
}