Social worker
Social worker
Sylvia the Social Worker and a baby
The Social Worker is an NPC who will take away a baby, toddler, or child if their needs drop too low. 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.
If The Sims 2: Seasons is installed, and a toddler or child becomes too cold or too hot, they will also be taken away. In The Sims 2, 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.
In The Sims 2, the Social Worker will also take away any babies, toddlers, or children who have been left home alone. She also gives children to families that have adopted a child. In The Sims 3, as in The Sims, children (but not babies and toddlers) can be left home alone. The Sims 3 actually makes it impossible 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.
Oddly, should another family obtain 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.