I was playing catch with my son and then I felt the urge to throw the baseball at his knee.
That's horrible, why would you intentionally hurt your child?
It is my only way to teach Them how to catch it correctly.
You wanted to teach your son a lesson, but just throwing a ball at his knee was pretty darn abusive. He could have a broken knee, the pain could have caused him such harm and sorrow. Next time teach by talking and not physically hurting.
I just wanted to make sure that he gets the message.
I get that, but there are many more ways of doing that that don't involve harming your child. Other than causing emotional damage to your child you may get in trouble if people notice this harm that you're doing