config.yml

sonata_doctrine_phpcr_admin:
    document_tree:
        Doctrine\ODM\PHPCR\Document\Generic:
            valid_children:
                - all
        Symfony\Cmf\Bundle\SimpleCmsBundle\Document\Page: ~
        Symfony\Cmf\Bundle\RoutingBundle\Document\Route:
            valid_children:
                - Symfony\Cmf\Bundle\RoutingBundle\Document\Route
                - Symfony\Cmf\Bundle\RoutingBundle\Document\RedirectRoute
        Symfony\Cmf\Bundle\RoutingBundle\Document\RedirectRoute:
            valid_children: []
        Symfony\Cmf\Bundle\MenuBundle\Document\MenuNode:
            valid_children:
                - Symfony\Cmf\Bundle\MenuBundle\Document\MenuNode
                - Symfony\Cmf\Bundle\MenuBundle\Document\MultilangMenuNode
        Symfony\Cmf\Bundle\MenuBundle\Document\MultilangMenuNode:
            valid_children:
                - Symfony\Cmf\Bundle\MenuBundle\Document\MenuNode
                - Symfony\Cmf\Bundle\MenuBundle\Document\MultilangMenuNode

if you are on the latest version of simplecms, its no longer Document\Page but Doctrine\Phpcr\Page