put them in the love tub get them to kiss a lot and look for the link that says try for a baby. Hope this helps I asked my 10 yr old daughter to help and will get her to give you more info later
In the Sims 2, the easiest way to get your female sim pregnant is to tell both sims to relax on a double bed, then click on the opposite spouse from the one you are playing and click the interaction try for baby.