Answer:
Get the two sims that you want to have the children, and make them both relax on a double bed.
Then, click on the sim which ur not playign and click try for baby or woohoo
If it isn't there you need to get the replationship higher, ususally the lifetime replationship should be about 70 or 80.
Your sim might not always get pregnant, but u will know because after the woohoo a little nursery rhyme tune will play.
Hope that helped, and keep playing :)
tmi