No, because the Portuguese sentence means that they intend to sit on the chairs, not that they are already seated. A more accurate (but less literal) translation would be "let's go sit on those chairs". If you wanted to say you were already seated, you would say "nós estamos sentados nestas cadeiras" (and yes, in this case it would make no sense to say "naquelas").
Isnt this a reflexive verb sentar-se? Then it should be (NÓS) NOS SENTAMOS. or can sentar-se be used(maybe not completely correct) without the reflexive pronoun?On that note,some brasilian teacher told me that to marry should be casar-se but colloquially people say just casar.