How to map children on a phpcr document. Children always empty

Question

i have a document 'Page' which should have different "elements" of unstructured content on the page.

The mapping of my document is loaded properly (i can create documents) and it looks like that:

<document name="Gondoliere\CoreBundle\Document\Page">

    <nodename name="title" />

    <field name="name" type="string" nullable="true" />

    <parent-document name="parentDocument" />
    <children name="contents" fetch-depth="3" />
</document>

I expect the $home->getContents() to return a collection with my items.

The node:dump --props looks like that:

Home:
- jcr:primaryType = nt:unstructured
- jcr:mixinTypes = Array(    [0] => phpcr:managed)
- phpcr:class = Gondoliere\CoreBundle\Document\Page
- phpcr:classparents = Array()
- name = Startseite
  687509281:
  - jcr:primaryType = nt:unstructured
  - jcr:mixinTypes = Array(    [0] => phpcr:managed)
  - phpcr:class = Gondoliere\CoreBundle\Document\SimpleElement
  - phpcr:classparents = Array()
  - headline = Willkommen
  - body = hier kommt mein Text

Somehow $home->getContents()->count() is always 0. What am I doing wrong. Page::getContents method looks like that:

public function getContents()
{
    return $this->contents;
}