make child interface neutral, support hierarchyinterface as well

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | yes
| BC breaks?    | only with regard to the previous RC
| Deprecations? | no
| Tests pass?   | travis
| Fixed tickets | #137
| License       | MIT
| Doc PR        | TODO: update admin doc and child interface infos

This would rid us of the dependency confusion we introduced with the ChildInterface.