L.T.
I don't know anything about endoscopies but my son(3 years old) goes through frequent medical procedures, mostly spinal taps, IV hook-ups and blood draws/transfusions. For his very first procedure (spinal tap and bone marrow aspirate) the doctor described who would be in the room, how they would numb injections sites, how they would sedate him, what instruments they would use, what recovery would be like, and the purpose for each of the procedures. Knowing the details helped me to remain calm and know what to expect as well as how to comfort and care for my son when it was over. If your child's doctor hasn't given you the details about the procedure you may want to call before your appt and ask.
I would make sure your daughter has plenty to eat and drink up to the cut-off time.
Take a few of her favorite things to comfort and entertain her - blanket, toy, book, etc.
Dress her in clothes that are easy to get her in and out of.
If you are allowed, take snacks and drinks for afterward - my experience is that a light snack after sedation is better than a meal.
Have a plan for how to deal with your daughter if she is sick on the way home - spread blankets all over the back seat area, have someone sit next to her with a container, etc.
Be sure that you eat something before the appt (without your daughter seeing you). You will want to be well-rested and well-fed so you can better deal with the events of the day and help your daughter through the experience. I tried to "stick it out" as long as my son had to and I ended up hungry and cranky and wasn't able to comfort him like I should have.
Is there someone who can go with you to keep you company and to help care for your daughter while you are getting discharge info, discussing results with the doctor etc? If your daughter is fussy afterward it may be hard to concentrate on any discussions.
Most kids seem to recover quickly from procedures. My son will be groggy for a few hours after his procedure, but by the next day he is back to normal.
I hope this helps alleviate some of your anxiety. It is hard not to worry, but being prepared and having information makes the experiences a little more tolerable. Good luck and be sure to let us know how it goes.