The key things I told my DD (who was 4) and have explained to any small child is:

The Dr's need your help to blow up a special party balloon (which is the gas they use to send them off to sleep)
When you're asleep, the Dr's are going to give you a special magic straw in your hand. It's very important not to play with this because it is magic. If you need special medicine, they will give it to you through the magic straw. (Most will put a bandage around the IV for kids).
When you're asleep, the Dr's will use a special camera to look in your tummy. But it won't hurt and you won't even know!

Other tips - send her special teddy/blankie.
Dress he in button up top as some places don't make little kids get changed
Take plenty of entertainment (for both of you!).
If she starts to cry, try and keep calm. Some kids won't partake in the blowing up the balloon theory - but if you cuddle them or stroke their hair and sing a fav song, they will stay still enough for someone (sometimes they let Mum/Dad do it) to hold the mask near their face until they drift off to sleep.
Keep in mind that one of the reactions of the gas is that patients twitch a little. Totally normal.

One of the drugs used has what is called an amnesic property. Which means you don't remember much (if anything) about going off to sleep or waking up. 75% of patients don't remember anything about the day at all. Although my monkey can tell me everything up to the point she fell asleep and everything from when I took her IV out (perk of being a nurse and having worked in that recovery room). DD lost about an hour in total from her memory.

To help avoid traumatizing her, we told her if she was well behaved, she could pick any balloon she wanted from the gift shop on the way home. Which she did. And we let her have McDonald's on the way home because she wouldn't eat in recovery.

It will be ok. Feel free to ask specific questions if you have them.

MG