@64173279 Well, kids that are taken away by social services vanishes from the game. If one acts really fast, there's a small window to get them back, but from what you're telling us, it seems that opportunity have passed, so unless you have an earlier save to go back to, the kid is gone.
If it just happened and you haven't exited the game you can try these steps, but no guarantee..
Select Save and go to Manage Worlds.
Select the same household.
Go to More options.
Click on Transfer Sims between households.
Select Not in world.
Select the baby that was whisked away.
Add the baby back to your household.
