Practice coughing and deep breathing. If your having tonsils out it will hurt to drink, but drinking is very important. Practice drinking and pretend it hurts. 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. Let your child explain it to friends and other family members.
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 doesn't help you or your child. This is general practice anyway and shouldn't be a problem. Stress to your child that no one will sneak up and do anything without your talking to him first.