Oh, yes, the lovely MRE. My son has had one every year for the last three years.
With ours...
No eating or drinking four hours before the test and we have to arrive two hours before the actual MRE. Each time he has had to drink 800cc of the barium contrast. Ours is served cold and supposedly fruit flavored, though DS would disagree. It has been suggested here that you can flavor it yourself with a little Kool Aid. I'd strongly suggest not using a straw because that just adds air to the stomach and fills you up faster. He'll need to drink as much as he can within an hour. This is where we've always struggled. My son has never been able to drink that much. He simply gets filled up and nauseous. Some kids can, some can't. Might be worth a try for your first time, but if you think he's going to be too anxious or really won't be able to drink it, I'd jump right to an NG tube.
An alternative to drinking the contrast is having an NG tube inserted and pumped in that way. This is my preferred method, though not my son's, because they get all the contrast in which allows for better images. We book a radiology appt and a dr inserts the tube, checks it via X-ray, pumps in the barium removes the tube and you're done.
Once the contrast is in, he gets a IV placed, then it's off to the MRE. It usually lasts about 45 minutes. My son is given headphones to listen to music and drown out the MRE jackhammering noise. Throughout this time he'll be holding his breath during imaging - sometimes as long as 20 seconds. It can be quite the workout, but it's nothing super hard or scary. Just holding his breath and tolerating the machine's noise.
Near the end, he most likely will be given a dose of glucagon via his IV. This might make him nauseous or feel funky. It's pretty common for people to throw up, but it passes quickly. Twice my son was fine. Third MRE he did throw up. Bring a change of clothes.
A little more time in the machine and then he's done.
They may cancel the test if he throws up the contrast or has too many BM's.
Afterward, he'll need to drink lots and lots of water to flush the contrast out of his system because worst case scenario, it can harden and cause a blockage. Lots of water!
Afterward my son is usually exhausted and naps for awhile. He's completely fine for school the next day.
Hope that helps!