To make a baby you relax on the bed with your boyfriend/girlfriend. Then press your boyfriend/girlfriend and it will come up with a few options:

Cuddle in bed
Chat in bed
Try for a baby
Make out and whoohoo.

Click on try for baby, do this a couple of times and soon your girl Sim will start to puke, then she goes pregnant, then labour, then you have your baby!

The next step is to go onto build and buy. Click on buy, then entertainment, then parties and it should have a cake there - buy it.

Go out of build and buy mode, then click on your cake and click blow out candles. It will give you the options of your woman Sim, your male Sim and your baby!

Click on your baby's name, then it will automatically change it to the toddler age.

Now buy another cake and do the same until your baby is to the age you want it!