Just do romantic stuff with the sim.
Chat with our AI personalities
i think soyes you can you have to be at least at the relationship of romantic interest.than you can ether relax on the bed with the other sim or talk to the and go to romantic.
You need a female and a male sim that have romantic relationships. Keep on being romantic until it says "(sim name) finds (sim name) extremely irresistible. then under romantic you will see try for a baby. or you can click on the bed and select try for baby.
Your sim needs to be best friends with the other sim before they can be in a romantic relationship. Once you sim's relationship is high enough with the other sim, your sim will have the option to propose.
I only have sims 3 for iPhone (swell as sims 3 ambitions) but I don't think you can have kids in just sims 3. As for sims 3 ambitions, I think you need to bemarried first and after a couple of other 'romantic' things it will become an option.
your sims will stay in love, so you just have to keep up your relationship (without using romantic interactions) until the teenage sim ages to be a young adult.