Rationality helps you map out the relations between actions and goals, and between goals and subgoals; and it can help us better understand the structure of the goals we already have. We can say that doing something is good because it helps achieve goals, or bad because it hinders them; and we can say that certain things are also goals (subgoals), if achieving them helps with our original goals. However, this has to bottom out somewhere; and we call the places where it bottoms out - goals that're valued in and of themselves, not just because they help with some other goal - terminal values.

Rationality has nothing whatsoever to say about what terminal values you should have. (In fact, those terminal values are implicit when you use the word "should".) For people who want children, that is usually a terminal value. You cannot argue that it's good because it achieves something else, because that is not why people think it's good.