We have this functionality in our road-map. But for now, you have to follow next steps.
In order to save the object with all the children in a single call you need to follow all the steps which are described in the document. It seems like, in the document that you sent us you skipped some steps. In order this works, you need to follow step 1-4 from this document: https://backendless.com/how-to-save-an-object-with-all-the-children-in-a-single-backendless-call.
Firstly, you need to download CodeRunner and model classes. Then, create a new project and Bootstrap class. After that define the service. And only after these steps you can save an object.