Jehosheba. Probably a half sister of Ahaziah — the daughter of Jehoram, not by Athaliah, but by another wife. She was the wife of Jehoiada the high priest (2 Chronicles 22:11).
Which were slain. Joash was taken, not from among the bodies of those already slain, but from among the princes who were doomed to death.
In the bedchamber. Not in the palace, but in the Temple quarters of the priests. Jehosheba would hardly have been able to keep the child concealed in the palace, where everything would be under the watchful eye of the queen. But in the Temple quarters she could be expected to exercise a large measure of control.