You can't have children unless you have a spouse, and pay some attention said spouse. This is the menu you use to do so. Each "attempt" takes one day of time and, of course, you must be in the same fief as your spouse when you make the attempt.

Depending on the Virility levels of you and your spouse, it may require many attempts. If the Virility levels of either, or both, of you is too low, conception will be impossible.