How can I make a stream of parent/child objects from firestore

I have a firestore database with a hierarchical relationship. The "Parent" document has a collection ("children") of child objects. I want to have a stream of Parent objections. So if the parent changes in Fire store the stream should offer a new Parent object with all the children loaded.

The code below is what I am trying to achieve. The problem is in the line marked "====> Problem", which in inserted into the code.

The compiler says : The return type 'List>' isn't a 'List', as defined by anonymous closure.dart(return_of_invalid_type_from_closure).

I don't know how to convert this stream to "map" call to an asyn call to get a List instead of a Future

Anyone know a good sample for hierarchical Firestore data in Flutter/Dart?

import 'package:cloud_firestore/cloud_firestore.dart';

final parentsCollection = Firestore.instance.collection('parents');

Stream<List<Parent>> jots() {
  return parentsCollection.snapshots().map((snapshot) {

 //====================================================
        return snapshot.documents //  <===== Problem
 //====================================================
        .map((doc) async { 
          var parentEntity = await ParentEntity.fromSnapshot(doc);
          return Parent.fromEntity(parentEntity);
        }).toList();
  });
}

class Parent {
  final String id;
  final String data;
  final List<Child> children ;

  Parent(this.data, {
    String id, 
    List<Child> children
  })
      : this.id = id ?? '',
        this.children = children ?? List<Child>()
    ;

  static Parent fromEntity(ParentEntity entity) {
    var parent = Parent(
      entity.data,
      id: entity.id,
      children: entity.children.map((c)=>Child.fromEntity(c))
    );
    return parent;
  }
}

class Child {
  final String id;
  final String label;

  Child(this.label, {String id})
      : this.id = id ?? '';

  static Child fromEntity(ChildEntity entity) {
    var child = Child(
      entity.label,
      id: entity.id,
    );
    return child;
  }
}

class ParentEntity {
  final String id;
  final String data;
  final List<ChildEntity> children;

  ParentEntity( this.id, this.data, this.children );

  static Future<ParentEntity> fromSnapshot(DocumentSnapshot snapshot) async {
    var children = await _childrenFromSnapshot(snapshot);
    return ParentEntity( 
      snapshot.documentID, 
      snapshot.data['data'], 
      children
    );
  }

  static Future<List<ChildEntity>> _childrenFromSnapshot(
      DocumentSnapshot snapshot) async {
    var childCollection = snapshot.reference.collection("children");
    QuerySnapshot docs = await childCollection.getDocuments();
    return docs.documents.map((doc) {
      return ChildEntity( doc.documentID, doc.data["label"]);
    });
  }
}

class ChildEntity {
  final String id;
  final String label;

  ChildEntity( this.id, this.label );

  static ChildEntity fromSnapshot(DocumentSnapshot snapshot) {
    return ChildEntity( 
      snapshot.documentID,
      snapshot.data['label']);
  }
}