The main issue here is that the original Greek uses a form that does not exist in English. The relevant portion of the Greek text reads:
Πάτερ ἡμῶν ὁ ἐν τοῖς οὐρανοῖς·
Translated directly, word for word, that is:
Father ours the one in the heavens.
Or, to make it closer to actual English:
Our Father, in the heavens.
The ὁ ἐν τοῖς οὐρανοῖς has no direct equivalent form in English, it literally means the one in the heavens. Well, the sky actually, but some poetic license is assumed. The main point is that there is no verb there, the text is not saying "who is in heaven" but "the one in heaven".
As @anongoodnurse already pointed out, the prayer addresses God in the second person, therefore, the art is quite correct and the sentence could be rephrased to:
Our father, thou who art in heaven, hallowed be thy name.