parent

Project.ParentModel = Backbone.Model.extend({
    initialize: function(args){
      // children is a collection
      this.set('children', args.children);
      this.listenTo(this.get('children'), 'add', this._onChildrenAdd);
    },
    _onChildrenAdd: function(child, collection){
      this.set('total', collection.reduce(function(memo, ch){
        return memo + ch.get('value');
      }, 0))
    }
  });

Project.ParentView = Backbone.View.extend({
    initialize: function(){
        this.listenTo(this.model, 'change:value', this._onModelValueChange);
    },
    _onModelValueChange: function(model, value){
        this.$('some container').text(value);
    }
})