I would suggest they check this child for tethered cord, sleep apnea,
ICP, ehlers-danlos,
and u mentioned no syrinx, but where did they check? Many Drs only look in the cervical spine, these can form in the thoracic and lumbar spine as well.
Last...8mm is the length of the herniation, but it is the measurement they do not give u that is more important, the width of the tonsils...u need to find out if there is over crowding and a CSF obstruction.