S.M.
My son had an upper gi at 4months. He did a Barium bottle not sure what your baby is going through but, basically this is what happened to us:
he was undressed down to his diaper we got to stay in the xray room with him and help position him and he drink his bottle. He was totally fine with all of it because he got a bottle. LOL Anyway, he drank and we positioned him and they took the xrays. We were in there for a total of 20 mins including undressing and dressing. Hope this helps. Good Luck.