Instructions
Load "The Sims 3" as normal. When in your game, hold down "Ctrl," "Shift" and C" on your keyboard then type "testingcheatsenabled true" to enable the game's cheat mode.
Select one room in your house to keep the kidnapped baby or toddler in. Add a crib to this room.
Hold down "Shift" on your keyboard then click on your mailbox. Select "Make me know everyone."
Visit the house of the baby or toddler you wish to kidnap. Introduce yourself to his or her parent. View your relationship with the parent then drag the relationship bar to the top of the screen.
Pick up the baby or toddler you wish to kidnap then pause the game. Move the camera so you can see your house. Hold down "Shift" on the keyboard then click on the floor of the room in your house that you wish to keep the kidnapped child in. Select "Teleport Here" from the menu that appears.
Unpause the game. The baby or toddler you kidnapped will now be in the room with the crib. If the youth's parents visit, remove the door from this room to prevent them from stealing him back.