child1_config.yml

services:
    application.child1bundle.doctrine.listener.inheritance_mapping:
        class: Application\BaseBundle\Doctrine\Listener\InheritanceMapping
        arguments: [ %application.child1bundle.discriminator_mapping% ]
        tags:
            - { name: doctrine.event_listener, event: loadClassMetadata }

parameters:
    application.child1bundle.discriminator_mapping:
        -
            parent: Application\ParentBundle\Entity\Category
            child: Application\Child1Bundle\Entity\Category
        -
            parent: Application\ParentBundle\Entity\Product
            child: Application\Child1Bundle\Entity\Product

child2_config.yml

services:
    application.child2bundle.doctrine.listener.inheritance_mapping:
        class: Application\BaseBundle\Doctrine\Listener\InheritanceMapping
        arguments: [ %application.child2bundle.discriminator_mapping% ]
        tags:
            - { name: doctrine.event_listener, event: loadClassMetadata }

parameters:
    application.child2bundle.discriminator_mapping:
        -
            parent: Application\ParentBundle\Entity\SomethingElse
            child: Application\Child2Bundle\Entity\SomethingElse

Listener.php

<?php

namespace Application\BaseBundle\Doctrine\Listener;

use Doctrine\ORM\Event\LoadClassMetadataEventArgs;

class InheritanceMapping
{
    protected $mappings;

    public function __construct($mappings)
    {
        $this->mappings = $mappings;
    }

    public function loadClassMetadata(LoadClassMetadataEventArgs $eventArgs)
    {
        if (empty($this->mappings)) {
            return;
        }

        $metadata = $eventArgs->getClassMetaData();
        $class = $metadata->getName();

        foreach ($this->mappings as $map) {
            if ($class == $map['parent']) {
                $metadata->addDiscriminatorMapClass($map['child'], $map['child']);
            }
        }
    }
}