How to listen only when child added !! not read all data

I have this code

var dataRef = firebase.database().ref('offers/');
dataRef.once('value').then(function (snapShoot) {
snapShoot.forEach(function (datum) {
    keys = datum.key;
    var newdataRef = firebase.database().ref('offers/' + keys + '/users');
    newdataRef.on('child_added', function (dataSnap) {
        Notify("SOME TEXT");
    })
   })
})`

and I used on('child_added') to add a listener at user's point at this data object here and unfortently this on('child_added') is working for the first execution with the entire data and act like it's on('value')

how could i make it truly listen only for child addition to this node