All You have to do is do a nested query .! first find key of parent then get pass that key to next query and then find its children.!
List<user> userList=new ArrayList();
         mdatabaseRef.addValueEventListener(new ValueEventListener() {
      @Override     
      public void onDataChange(DataSnapshot dataSnapshot) {
         for (DataSnapshot child: dataSnapshot.getChildren()){
            String key = child.getKey();
             fun(key);
             }
      private void fun(String key){
        mdatabaseRef.child(key).addValueEventListener(new ValueEventListener() {
       @Override     
       public void onDataChange(DataSnapshot dataSnapshot) {for (DataSnapshot child: dataSnapshot.getChildren()){
    User user= postsnapshot.getValue(User.class);
userList.add(user);
     }
        }
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solved how to retrieve all the keys in firebase android