Remember, be positive. Making preparations ahead of time for after surgery reinforces to the child he will live. Practice, Practice, and Practice Practice your signs while on the respirator. Practice coughing and deep breathing. The more you practice, the easier it will be for your child when the real thing happens - knowing what to expect and how to act you will be giving your child tools that can be used in coping with things for the rest of their life.
Kids are pretty matter of fact.
I think we can learn from them sometimes. The night before surgery, keep yourself and your child busy. Sitting around worrying does not help you or your child. This should be a general practice anyway and should not be problem. Stress to your child that no one will sneak up and do anything without you talking to him first.