Is it possible to distinguish "Do you have children yet?" from "Do you already have children?"? Both are counted as correct here, but they have different meanings in English.
"Do you have children yet?" means that the speaker is expecting you to have children at some point and wants to know if it's happened.
"Do you have children already?" suggests that the speaker is not expecting you to have children at this point.
The dutch sentence can mean both but. Heb jij nu al kinderen? Would mean that the speaker is surprised (and maybe disapproving) of the possibility that the listener has children.
In both sentences the speaker expects children to happen at some point. Which is a risky assumption to make. The question: Heb je kinderen? (Do you have children) Is far safer.