如何在Firebase中获得孩子的价值？

我正在尝试从数据库中获取riderId值：

我正在使用的代码：

Query query = FirebaseDatabase.getInstance().getReference(Common.user_rider_tbl);
            query.addValueEventListener(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
                    riderId = String.valueOf(dataSnapshot.getChildren().child("riderId"));
                    Log.d(TAG, "riderId = " + riderId);
                }

为了获得riderId的价值，我做错了什么？

json

"Riders" : {
  "C0RjB5NPZcTvWz9XiUAhpTDOK0C2" : {
    "avatarUrl" : "https://firebasestorage.googleapis.com/v0/b/ryyde-sj.appspot.com/o/images%2Friders%2F0badce3c-e75b-4d52-88e1-d2009090fb73?alt=media&token=5ca59575-54ce-401e-ac27-c1d4e6a5f94c",
    "email" : "[email protected]",
    "name" : "Rider1",
    "password" : "zzzzzz",
    "phone" : "665",
    "rates" : "0",
    "riderId" : "C0RjB5NPZcTvWz9XiUAhpTDOK0C2"
  },