My child is competing in a sports event near Tokyo at the end of July, so we've decided as a family to go extra early and spend time touring beforehand. We have 9 nights and 8 days for this, though the last day we will need to be getting settled earlier in the day/afternoon at the competition hotel. We will base ourselves in Kyoto for 4-5 nights and want to see Hiroshima as well.
Two questions:
1. Should we just do a day trip from Kyoto to Hiroshima or turn it into a destination?
2. The last 2 days or so needs to be chill, because kiddo shouldn't be doing strenuous activity or be on their feet all day in prep to compete. Is it feasible to book a relaxing night or two from Kyoto to Kawasaki area? Like Izu Peninsula or near a body of water (we are landlocked where we live and live near mountains so the water/sea is extra interesting to us).
Basically, just asking about how to organize things, keeping in mind we are a family with 3 kids, ages 16-21, active and fit, probably will be willing to walk forever in the first 5 days or so. Also consider kids being "temple and shrined out" vs myself who could do that forever. We can split up as well given the ages. Thanks so much for any itinerary or ideas.

It will be very humid and hot at the end of July in Tokyo. Highs around 30 although 40 occasionally happens too if you are unlucky. I doubt you will want to walk for hours and hours anywhere. Izu is is about an hour south of Tokyo and not close to Kyoto. It is a popular summer destination for the Japanese so you should book soon. Helps to have a car in Izu

Re: Hiroshima. Your call! It's easy enough to get there from Kyoto (see link below) and you would have enough time to see the main sites (Peace Park and Peace Museum). Whichever you choose, I would be sure to plan a quiet evening after visiting those sites -- the experience can be quite emotional.
https://www.japan-guide.com/e/e3404....FROM=2158_2160

If you haven't already consulted it, I'd urge that you spend a lot of time with japan-guide.com -- it is a truly awesome source of information about travel in Japan.

The sun can be punishing in Japan during the summer, However, with a wide brimmed hat or a parasol, or out from the sun due to cloud cover, etc summer in Japan can be pleasant even with physical activity or being outdoors for prolonged periods. Just be prepared. YMMV of course and everyone can be affected differently.

Someone recently posted about taking a ferry from Hiroshima to Miyajami Island and staying the night. Not sure if you have time for that.....

Miyajima is lovely! I think you'll find MANY posts about that island on this board if you search for it -- or plenty of info on japan-guide.com or in any competent guidebook.

A day trip from Kyoto to Hiroshima is doable, but it really depends on what you plan to do. If you're heading to Miyajima Island, you likely won't have time to explore other spots in Hiroshima city.

>> near a body of water

Japan is not a beach destination but consider:

2. Kujukuri Beach, Chiba

3. Isshiki Beach, Kanagawa

4. Shirahama Beach, Shizuoka

described on this list:

The 8 Best Beaches in Japan, From North to South | Rakuten Travel

Am aware that beaches do get packed.
Maybe something else around Kamakura.

More info here:
Best beaches in Japan