Hi D.,
My daughter, who is 5 months now, had one done two months ago. She was breastfed exclusively, but spit up very frequently, stopped gaining weight, and would stop mid feeding screaming in pain. We did the barium swallow and an ultrasound to determine if the problem was reflux or an anatomical abnormality. The barium swallow procedure was really very easy. She couldn't eat for 4 hours before (I would suggest scheduling the test for first thing in the morning, to hopefully avoid waiting due to the delays that build up over the day). They gave her a chalky solution to drink (you'll need to bring a bottle) and took a series of images, waited 15 minutes and took some delay images. She was reclined on the imaging table the whole time, and was not at all uncomfortable. I was present the entire test, you should be able to accompany your child if you wish to. My daughter's test was positive for reflux, she's now on Zantac and eating like a champ and gaining weight again.
I do want to mention that the next day after the barium swallow her tummy was quite unsettled, gurgling and growling all day. She was also more fussy than usual. The pediatrician did say the barium can cause that as it passes through the bowel, that was normal. It might also cause them some diarrhea and chalky colored stools.
I'm sure you might be concerned about the radiation dose to your child from this procedure. I'm a nuclear medicine technologist, and familiar with medical procedures involving radiation, and I felt that the benefits from an accurate diagnosis were well worth the risk. The techs performing the test should use shielding under your daughter to protect her ovaries.
One additional tip: no doubt your daughter will be hungry before the test. You can take a pacifier and dip it in sugar water for her to suck on. It is amazing how this will calm them down and it won't interfere with the restrictions of not eating before the test. It works wonders.
Good luck!
S.