In the context of Hosea 1:2–9, the most obvious answer is the people of the northern kingdom, those whom Hosea had just labelled “children of whoredom” (Hosea 1:2) and “Not My People” (Hosea 1:9). This term is clearly used this way in Hosea 1:11, since Hosea mentions the children of Judah separately.1 But at the same time, the fulfillment of this promise must go beyond the historic people of God in the north. (see for instance the reference to Judah in the next verse, Hosea 1:11).
10 Yet the number of the children of Israel shall be like the sand of the sea, which cannot be measured or numbered. And in the place where it was said to them, “You are not my people,” it shall be said to them, “Children of the living God.”