Instructions
Build the relationship between the other Sim in your household. Use conversations, group activities and a lot of interaction to reach the "Love" status. After a few "Sim Days," one of the characters will express interest in a child and eventually you will receive one.
Follow the wants and needs for the newborn baby. The basic needs are playing, feeding, naps and diaper changes. After a few "Sim Weeks," the baby will transform into a young child.
Keep the relationship thriving with the other Sim. Wait for the "Baby" message to appear on the list of options. Select this option and you will receive another newborn. Repeat the same baby-raising techniques until two children are in the home.
Send the kids to school on a daily basis. Keep their grades up and make sure they study at home to build their skills. Eventually the children will graduate school and become young adults.
Control one of the children Sim characters and have it call young adult neighbors. Build relationships with one of the neighbors. Continue that relationship until the two characters are in love.
Ask the other Sim if your child can move in with it. Once that Sim moves away from home, take your original Sims, build on their relationship more until you can create another baby. Repeat this process for as many children as needed.