The coalition of Syria and Israel is referenced by using their capital cities to stand for the country and their kings. Before the child reaches the age where he is able to call for his parents by name, destruction will be upon Syria and Israel at the hand of the Assyrians.
Literary analysis: Having the timing of the prophecy tied to the child calling for his mother and father is a nice reinforcement of the parentage of the child in the previous verse. When this particular child calls, he calls not only as an infant, but as the bearer of prophecy.