The sentence with "das" is right.
To explain, note that "das" is not an article in this case, so it is not "das Kaengurus", that would indeed be wrong. Instead, here "das" is a pronoun that can be translated as "that", as in
Are that kangaroos in the background?
For a long list of examples of der/die/das as a pronoun in German, you can look at the Duden.
If you want to use an article here, that would be
Are that the kangaroos in the background?
Sind das die Kaengurus im Hintergrund?
This implies that you already know (of) the kangaroos, e.g. because you mentioned before that you have seen some on your trips.