followingList = new ArrayList<>();
DatabaseReference reference = FirebaseDatabase.getInstance().getReference("Users")
        .child( FirebaseAuth.getInstance().getCurrentUser().getUid())
        .child("productos");

reference.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        followingList.clear();
        for (DataSnapshot snapshot : dataSnapshot.getChildren()){
            followingList.add(snapshot.getKey());
        }
    }

    @Override
    public void onCancelled(@NonNull DatabaseError databaseError) {

    }});

"productos" : {
"-LaSV9Nxwrz58Oe8sLwE" : {
  "-LaQoiF-_v_1BsEiqU7v" : true
},
"-LaSXD7-B6E51QRMMoWp" : {
  "-LaQoiF-_v_1BsEiqU7v" : true
}