DatabaseReference ref = FirebaseDatabase.getInstance().getReference();

        ref.child("Messages").child(sendersID).child(RecieversID).addValueEventListener(new ValueEventListener() {
          @Override
          public void onDataChange(DataSnapshot dataSnapshot) {
            for(DatasnapShot snapshot: dataSnapshot.getChildren()){
               snapshot.child("Seen").setValue(true);

            }
          }

          @Override
          public void onCancelled(DatabaseError databaseError) {
            System.out.println("Fallo la lectura: " + databaseError.getCode());
          }
        });