The prophecy of time involves a child, just as in chapter 17. Verse 18 below clearly notes that Isaiah and his children are messengers of the Lord's will. Thus contextually we are dealing with second child of Isaiah, another born with an auspicious name, and serving as a marker of time for the fulfillment of prophecy.
Literary analysis: Isaiah is consummate poet, and it would be folly to suppose that any word a important as "prophetess" would be accidental. What reason is there, then, for the word "prophetess" rather than "wife?"
Isaiah is a prophet. He is issuing a prophetic proclamation from God. The union with a prophetess reinforces the prophetic meaning of the child. From both father and mother, the son will carry the message, and become the embodiment of prophecy. That the prophetess was Isaiah's wife is of lesser import than the symbolic force that the prophesied child of two prophets would have.