You will have a child if you completed the whole game's plot and if you're married. Regardless on who you marry, they will ask whether you want a girl or a boy. Only their hair and eye color will change depending on who you marry. And their skin color will change only if you marry Maerwen.
To have a child, talk to your husband/wife everyday for a month and then they'll ask you to stop by the clinic. After 2-3 weeks they'll ask if you want a boy or a girl (be sure to talk to them) and in another week the child will be born.