Who was hidden from Athaliah as a baby?

Answer: Joash

Jehosheba took Joash and hid him from Athaliah so he was not killed.

“But Jehosheba daughter of King Joram, the sister of Ahaziah, took Joash son of Ahaziah and stole him away from among the sons of the king who were being murdered. She put him and his nurse in a bedroom to hide him from Athaliah, and he was not killed.”— 2 Kings 11:2 (BSB)