My son w/CF is 3 years old and had a chest and sinus CT July 19. It was done right after he had a PICC line inserted so he was still woozy from sedation. If he had not been sedated, I doubt he could have stayed still and not fussed about the CT procedure.
The machine they used was a giant donut-shaped thing and my boy had to lay strapped to a small table with arms held to his side and with foam blocks holding his head still. When the machine started up, the table moved back and forth into the donut. The inner part of the donut spins around and there is light and sound (about comparable to a fast fan running). Given that all of this is happening in a somewhat dark room full of machinery I can't imagine a typical tot staying still for it without at least a dose of benadryl to quiet him/her down. Both I and my spouse were allowed to stand next to him but could not hold his hands because they had to be kept out of the way when things were moving.
It was fairly quick to run the scan, but he had to stay put for a tough 15 minutes afterwards while the technician/nurse waited for the Radiology Dr. to come by and look at it. The boy was waking up by then and pretty antsy to get out.