Now I owe my life to the work of neurologists so I can help you a bit with that, I hope.
As I was in a teaching hospital, I had students trouping through and practising on me too!
The bulk of the testing we for reflexes - you have probably had those already, and also testing for sensation with cotton wool and, as I am pretty numb, they worked up to a pin which I could barely feel. Then there were Ishihara tests on colour blindness, and other tests on vision eg following a pin head moving around, and then towards me and away. I have a dim recollection of some hearing things but I didn't do enough that was interesting on those to have them repeated.
There was plenty of checking limb function and balance which is my main issue.
The big tests were the MRI scans, which I have no problem with, but you have to keep still. So not sure if they would do that for dc.
I think you are right to look for answers. Early intervention is so often the name of the game, but they need to have a crack at working out what is going on, so they can optimise treatment/therapy.
My dd had to have blood taken when she was little and they gave her skin anaesthetic so it didn't hurt. She didn't even flinch. And I understand that a whole gene sequence can be mapped just from saliva anyway.
Hope this helps