Lived in SE Asia for 10+ years and my wife is Japanese (just recently back in Europe)
. Tokyo is a exceptional city with a Disney World that might appeal to your kids depending on their age. However in summer the weather is not the best very hot and humid.
I would do a Thailand Vietnam combo doing perhaps Samui or Phuket + Bangkok together with Hanoi and Danang.
Hong Kong not my favorite city been there on business many times just wanted to leave asap but that is my personal view of course.