Oh wow, I'm not really the person to ask as most of my research has gone into my own condition.. I was hyper thyroid because of an over-active nodule that was producing too much hormone which, I'm pretty sure, would not relate to children.
I do know of transient thyroiditis which can have many causes (perhaps that's what your specialist believes it is) and, of course, Graves disease. I googled 'juvenille hyperthyroid' and there was an array of results.