Social worker

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The Social Worker is an NPC who will take away a baby, toddler, or child if their needs drop too low.

The Sims
In The Sims, the Social Worker will remove babies who have been left alone or left to cry for too long, but will not remove other children.

The Sims 2
In The Sims 2, in addition to removing babies, toddlers, and children whose needs fall too low, the Social Worker will take any who are left home alone, as well as children who are getting an F in school. If The Sims 2: Seasons is installed, the Social Worker will also come if a toddler or child becomes too cold or too hot. Unlike The Sims, if the Social Worker takes away a child, all other children under Teen will also be removed from the house.

Once they are taken away, they will all be available for adoption by a different family (or families). Once a person has the memory of having a child taken away by the Social Worker, they are ineligible for adopting ever again.

The only way to stop the Social Worker from taking away children without cheats is the "Plead with Social Worker" interaction, which is only available in FreeTime to Family Sims via lifetime aspiration benefits. Even if there is a wall around the children, the Social worker will teleport the child out of the structure and to her car. If she can't reach the child, then she will use telekenesis and the child will appear in front of her.

Visits from the Social Worker are not always negative, as she also delivers adopted kids to their new families.

Oddly, should a family adopt a child taken from another family, the child Sim will have lost all memories associated with their blood family up until the point they arrived in their adoptive home. The family tree does not get erased, however.

The Sims 3
In The Sims 3, as in The Sims, children (but not babies and toddlers) can be left home alone. The Sims 3 requires players to call a babysitter or stay home when a sim is about to leave babies and toddlers home alone; if the player makes all the Teen or older Sims go past the mailbox, they will be required to either go home or call a babysitter. If a baby or toddler is left home alone with a child, a babysitter must still be called. This is probably because the child is not capable of holding the baby/toddler, feeding it, changing its diaper, etc. However, there are still ways of leaving a baby or toddler at home completely alone. Leave one teen (or adult) at home and make them leave the house by either switching their household (via Edit Town) or make them leave household via a Miner Hole.

Getting rid of the social worker
If playing The Sims 2 or The Sims 3, and a Social Worker arrives on the lot to collect the children, players can use the (boolprop) testingcheatsenabled cheat to delete her before she has a chance to take them. It is unknown, however, if this action has any negative effects on the game. If you delete her car, she will be forced to leave. You can hide all Non-playable-characters (NPCs) by using the testingcheatsenabled cheat and shift-clicking your sim. Spawn the NPC and Townie maker, and find the "Hide All NPCs" button. This will also let you make NPCs and townies.