Hello! We spent 3 weeks there last summer, end of June to mid-July, and although I much prefer heat to cold, I never thought it was that hot. Apparently it got a lot hotter later in the summer though. I was told not to go in the rainy season, but it didn't rain everyday, and when it did rain, obviously it wasn't too hot. The nice thing about summer is that you can pack really light. Also it's a great time to try all those flavors of ice cream, and cold drinks from vending machines. If you go in the summer, make sure you get a/c in your accommodations. You know your kids, but kids can be pretty flexible. I grew up in Texas so it has to be pretty hot to get my attention, buy my son didn't, and he was fine.

I was in Tokyo in August 2017 and it was very, very hot. I am also from So California and know hot but I was just wiped out for a few days. We left the hotel early and tried to do things then, and then again in the evening.

We traveled down to Fukuoka and it was a bit better. This year, I am going back again in August, but mostly in Kyushu and then flying up to Hokkaido for a few days so I will miss the Tokyo heat. I would not cancel a trip or not go because of the heat, just pack extra clothes knowing you will be changing twice a day and plan what you can do if it is unbearably hot when you are there.

Since your vacation starts July 5, I would go to Tokyo and other hotter areas first and end the trip in August in Hokkaido and the Japan Alps where it is cooler.