Do Expats need supplemental health insurance? (Tokyo)

Hello - My family of 4 is moving to Tokyo on a 5-yr visa and I would love some insight into how health insurance works in Japan. What is covered with the Japanese health care system? Any suggestions for where to go for supplemental coverage?
In particular, one of my daughters needs physical and occupational therapy (developmental delays, not due to an injury) . . . does anyone have any experience getting those services covered by health insurance in Tokyo? Thank you for any suggestions!