This is a handler for the JMS serializer to handle parent and child type serialization. You need to set the type of your collection as `@Type("\Acme\MyBundle\Model\ChildType")`, so that the serializer picks up the handler correctly. Also remember that you need to hook up the handler as a service (and give it the correct tag).

CustomHandler.php

<?php

namespace Acme\MyBundle\Serializer\Handler;

use JMS\Serializer\Handler\SubscribingHandlerInterface;
use JMS\Serializer\EventDispatcher\Event;
use JMS\Serializer\GraphNavigator;
use JMS\Serializer\XmlSerializationVisitor;
use JMS\Serializer\GenericSerializationVisitor;

use Acme\MyBundle\Model\MyCustomParentType;

/**
 * Custom Handler for Parent and Child Types
 */
class CustomHandler implements SubscribingHandlerInterface
{
    public static function getSubscribingMethods()
    {
        $methods = array();
        foreach (array('json', 'xml', 'yml') as $format) {
            $methods[] = array(
                'direction' => GraphNavigator::DIRECTION_SERIALIZATION,
                'format' => $format,
                'type' => 'Acme\MyBundle\Model\MyCustomParentType',
                'method' => 'serializeTo'.('xml' == $format ? 'XML' : 'Array'),
            );
        }

        return $methods;
    }

    public function serializeToXML(XmlSerializationVisitor $visitor, MyCustomParentType $object, array $type)
    {
        if ($object instanceof \Acme\MyBundle\Model\ChildType1) {
            return $visitor->getNavigator()->accept($object, null, $visitor);
        } else if (...) {
            ...
        }
    }
    
    public function serializeToArray(GenericSerializationVisitor $visitor, MyCustomParentType $object, array $type)
    {
        ...
    }
}

$visitor->getNavigatior() is deprecated