I've had one. I'm 19 though. But for me it wasn't all that bad. I have a very strong gag reflex but i honestly didn't find it that bad. I had to drink a cup full of the barium mixture for the meal. Then for the swallow i had to hold it in my mouth and not swallow it until he asked me to (not sure how this would work with a baby). It makes your mouth feel really dry and gritty so make sure you have a drink ready for them as soon as it's done. Second thing i'd make you aware of is that the barium is heavy, so don't be surprised if your baby is grouchy afterwards, it makes your stomach feel very heavy and uncomfortable. I looked about 5 months pregnant after mine. All that said, babies will need a lot less of the barium mixture i guess, although it probably still will make them feel uncomfortable. Oh, and i almost forgot, their poo will turn white and it will be very heavy!
(my barium stuff was for stomach/digestion problems to diagnose severe reflux, will prob be different depending on what it's for)
Hope that helps a little.
xx