Hi and welcome to the Chiari forum.
First, know it is not the size of the herniation, but if it is creating a CSF obstruction so a CINE MRI should be done, as well as MRI's of the complete spine.....and a sleep study should be done to rule out sleep apnea...especially with children as they do not know how to explain if they have sleep disturbances.....then there is testing to rule out ALL related and non related conditions....
The list of Drs in posted in the Health Pages -
http://www.medhelp.org/health_pages/list?cid=186
The list is not a referral nor an endorsement....but a starting place for u to research Drs. the list was compiled by the members here of Drs they have been to, treated by and liked....what works for one may not for another, so research, educate urself and see a few Drs and compare them and go with ur gut.
Know u may have to travel to get to a true Chiari specialist, and there is a difference btwn a NS and a Chiari specialist.
For what to ask, see also in the health pages ?'s for the NS......
But keep an open mind, write down what is said, and ask how to ask more questions once u get home and digest what all was said...some Drs will give an e-mail addy so u can ask more questions.
Knowing they the Drs should look at the entire spine to rule out a syrinx, and not just the cervical spine, and know that surgery does not cure this condition or fix everything symptom wise....if a Dr says ur child will be cured, leave....as they do not have a clue......
Good luck and do keep us posted on how the visit goes.