I've only ever had them done with infants, so I don't know how many, if any, differences there will be with older kids.
For us, they drank from a bottle filled with barium while laying under a giant scanner. The size of the machine has the potential to freak an older child out, so you may want to talk about that in advance, but it doesn't touch them at all. There's a video screen that shows the barium making its way through the system, which they may or may not be in a position to watch, themselves. If they are, it's pretty cool!
You will be asked to wear a full apron and thyroid cover, so that's another thing you might want to talk to them about, since it can look awfully weird!
Hope that helps!