What are the minimum tests needed to diagnose mitochondrial disorder?

A good doctor. Mitochondria have their own dna, so mitochondrial gene mutations must be diagnosed at a lab such as dr. Boles at usc; but diagnosing mitochondrial dysfunction is a matter of appropriate interpretation of symptoms, as well as testing things such as vitamin B12 function (mma), mg (rbc), zn (rbc), and phase angle on body composition analysis. Find a doctor who knows mito disorders.
This is a long road. By the time these kids are labeled, they have been through a variety of workups with pediatricians, neurologists, etc. Early on this can mimic a seizure disorder, congenital infection, or other condition. With luck, they will end up in the hands of some of the few doctors in the country that can set up the proper testing, much of which is only done in a few labs.