This will be my family’s (husband and teen son) first trip to China from US, via Japan.
Please check if the itinerary makes sense, it is reasonable or not, if I should reconsider some of the places or amount of time spending in each. I know it will be hot, but that’s the time period we have available. There is the possibility I could add 2 extra days to this trip, if so, where should it go? Thanks for your advice!
June 25-28 (3 nights) in Hong Kong
June 28-July 2 (4 nights) in Zhangjiajie National Forest Park
July 2-July 5 (3 nights) in CHongqing
July5-July 10 (5 nights) Chengdu +Mount Emei or Qingcheng
July 10-14 (4 nights) in Beijing
July 14-17 (3 nights) in Singapore
Thanks!

I thought a week insufficient for Beijing. I didn't visit any of the other places you list for China.

I thought 5 nights barely sufficient for Singapore.

BUT it really depends on your interests. It would help if you would provide that information.

You might find some useful information in my trip reports:
China: I started and ended in Beijing.
Thanks for helping make my trip to China amazing!

Singapore: Near the start of this report:
A Memorable Solo Month in Singapore, Java, and Bali

My teen son and I went to China in Summertime, and I can tell you what I did to escape the heat. We went to Northwest China. We had nothing but clear skies and mild temps. No humidity. We spent the first 3 nights in Beijing, and the last 3 nights in Shanghai. I had already been to Hong Kong.

In between, we arranged for a car and private guide for 8 nights. Flew into Lanzhou and visited Bingling Si, Xiahe, Labrang Monastery, Sangke Grasslands, Langmusi, Juizhaigou, Huanglong, and Songpan. We also visited Japan before that.

When we got to Bingling Si, we had that amazing place all to ourselves. So it was a shock when we got to Jiuzhaugou, where the walkways were so overcrowded I was surprised nobody got pushed into the water. We hadn't seen that many people in a week.

For your itinerary, you are hitting some amazing spots I am sure, though I haven't been. Chongqing seems like an outlier, though. What pulls you there? I suppose most folks would head to Xian instead.

No matter what, you'll have a wonderful journey. You are certainly covering some great cities, natural marvels, and cultural sites. The one thing you are missing is rural village life, which was a priority for me.

BTW, Provincial Chinese websites can provide many ideas. Here is the one for Hunan Province, where you are planning to go: https://www.enghunan.gov.cn/Tourism/ I could spend my entire trip in Hunan alone.

Add them to Beijing. 4 nights is a bit tight for the Great Wall, Forbidden City, and Summer Palace, especially if the summer heat slows your pace down.