Use ChildInterface

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony-cmf/symfony-cmf/issues/188, https://github.com/symfony-cmf/MenuBundle/pull/177
| License       | MIT