Cant get my pet back from the social worker!

Don't think there's an option to get it back once it was take by the social worker. You will need to adopt another one or create a new one.

Do you have a save game prior to the pet being taken away? In general always good to make copies of your games in the event something goes wrong and you need to go back to a previous save.

Go to your gallery and place the same family the transfer the pet to the current family then delete the rest