Are there any good workouts to strengthen your chin to reduce the probability of getting knocked out?
You can strengthen muscles around the face like chewing alot, or you can wear a tight chin strap and open your mouth alot. Neck strengthening as well might help out with putting a weighted helmet on and then nodding up down, side to side, left to right, and circular motions with the heavier weight.
Box and get hit more with lighter hits and increase until you can take more hits. Although this will also have a long term affect as well, so you likely want to do both.
You'll need to train two things: neck and jaws. Neck is to keep your head more stable, so it doesn't get knocked around as much, jaws to take the brunt of the punch.
Neck can be done by standing in pushup position, putting the top of your head on the floor, and carefully rolling back and forth. Eventually, you can do this with your hands on your back, but at first, keep your hands on the floor.
The jaw is basically just clenching. The harder you clench your teeth, the more massive it is. It's the same principle as when you have your arm in a 45° angle, or closed up entirely. It gets moved around alot easier at an angle. If you get hit while firmly clenching your teeth, chances are your opponent's fist will hurts. If you don't clench at all, your jaw could get dislocated. This is more a matter of habbit than of practice, though. While warming up, some exercises require tilting your head back and forth, sideways, and so on. Clench your teeth as hard as you can there, so you train to get the reflex to clench your teeth when your head gets knocked around.
There's actually a third part too: Just getting knocked in the head. Mutt already covered this part perfectly in his answer.