I'm going to o against the grain, my DS was not pre warned as we would have not t to the surgery. Let alone get the injection. We did the check up 1st and spoke then about the injection with the nurse and the doctor he was much more settled this way and while he did cry he didn't fight it... Normally they would need to wrap him in a blanket and hold home down plus I would have to drag him kicking and screaming for a needle. So I would have been really stressed from the get go. This really was the best option for us! You know your child best though.
Good luck