The first sentence. "Them" is an indirect object because you aren't sending "them", you're sending the postcard. Also, you can only have an indirect object when there is an action verb (sent) and when there is a direct object (postcard); linking verbs such as "is" and "are" do not have indirect objects or direct objects. Hope this helps!
The sentence that uses the word them as an indirect object would be Leo sent them a postcard from NASA. Hope this helps and good luck with whatever it is you're doing!
