Get them to keep kissing and hugging and increasing how much they love each other. Then there will be a box that pops up and says 'Would you like to have a baby?' and then click yes and a baby pops...
um i belive all you have to do is get a boy and girl sim and keep kissing till she or he says "should we have a baby" then push yes or no but that's a guess you should try iT!