follow the property access model with children

| Q             | A
| ------------- | ---
| Bug fix?      | yes (well, workaround for sonata admin shortcomings)
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

with this change, routes will appear again in the sonata phpcr-odm as this follows the property access model of having getChildren to get the children property.