gistfile1.js

var ItemModel = Backbone.Model.extend({

    defaults : {
      "item_id" : 0,
      "item_content" : "this is a content"
    }

  });

var ItemColl = Backbone.Collection.extend({
    model : ItemModel

  });

var ItemParentModel = ItemModel.extend({

    initialize : function () {
      this.set("childCollection", new ItemColl());
    }

  });

var ItemParentColl = ItemColl.extend({

    model : ItemParentModel,

    toJSON : function (options) {
      return this.map(function (model) {

        var data = model.toJSON(options);
        data.childItems = data.childCollection.toJSON();

        delete data.childCollection;

        return data;
      });
    }

  });

// ――――――――――――――――――――
var mainItems = new ItemParentColl();

mainItems.add({
  item_id : 2121,
  item_content : "Item 1"
});

mainItems.at(0).get('childCollection').add({
  item_id : 2122,
  item_content : "Item 2",
  item_parent : 2121
});

mainItems.add({
  item_id : 2123,
  item_content : "Item 3"
});

mainItems.at(0).get('childCollection').toJSON();
// this has values

mainItems.at(1).get('childCollection').toJSON();
// this is empty

mainItems.toJSON();
// this shows the childItems as JSON items