cleanup return values and relax the addChild method

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | travis
| Fixed tickets | -
| License       | MIT
| Doc PR        | https://github.com/symfony-cmf/symfony-cmf-docs/pull/496

stumbled over these inconsistencies when checking https://github.com/symfony-cmf/symfony-cmf-docs/pull/496