The investigations I had mentioned, like neurophysiological studies, muscle biopsy, blood potassium levels during the attack do give the diagnosis almost certainly. Since MS has been ruled out, the other conditions which can mimic the clinical manifestations like chronic inflammatory demylinating polyradiculoneuropathy, hereditary sensory and motor neuropathies, thyroid function abnormalities, myasthenia gravis, Conn syndrome, hyperaldosteronism, should also be ruled out.
The periodic paralysis has usually three-four different varieties of presentation. One of them do have fixed muscle weakness. This muscle weakness usually do not increase after a certain duration. Another variety called as Hypokalemic periodic paralysis, if untreated, fixed proximal weakness is experienced, which may interfere with activities. But in this also, after the treatment is initiated, the muscle weakness does not progress.
I can understand your concerns as a mother. Any progressing muscle weakness has to be addressed and the diagnosis should be revisited. The other diagnosis, I had mentioned above should be looked for and should be ruled out or ruled in. A neurologist and an endocrinologist should work in tandem to get the best care.
Hope this helps.
Please feel free for your follow up questions.