How to map sub collection to json_serializable nested class?

Question

json_serializable supports nested classes:

https://docs.flutter.dev/development/data-and-backend/json#generating-code-for-nested-classes

My goal is to have a Parent class with a Child member, and I want the firestore to look like:

Collection(Parent) -> {parentId} -> Collection(Child) -> {childId}

What is not clear is how the firestore that backs this model is supposed to be structured and accessed.

Any ideas? Thank you