Both times I had an MRI it was splint free. I have metal bracketing in the middle of my splint so that would be REALLY bad. Not to mention, they need to know where the teeth sit in position and the range of motion, so definitely no to the splint.
Hope this helps,
Andy
PS. Tell your kid to make songs out of the bumps and thumps.. that's what I always did and it made it a little more fun.