create a link to first child page in menu

I have a small problem here and maybe there’s someone who can help me,

I have a menu, which just has a textlink to the first sub-page of page X, at this moment i have hardcoded this link. Is it possible with a line of code to automatically create a link to the first child of page X? The text of the link itself is just hardcoded as well, so the only thing that has to be generated is the link in the href