You should have your own collection containing the data that backs the markers and then a View for the collection which contains a View for each child. The child views will render themselves on the google map as a map marker. This way when a child updates the view can easily update the map marker (i.e. change its position on the map). Also, when the collection containing the children changes the parent view can easily add/remove child Views (i.e. the markers).
You can provide the Backbone Map view to the collection view which can then provide that view down to the children views that it creates. You will need to add a getMap() method or something similar to the Map view so that the children can get the Map to add themselves to.