Travelling to China in middle of August. What about the weather of Yinchuan, Dunhbuang and so on?

I agree with you. It would be hot taveling to these places, especially Dunhuang and Linxia.
Yinchuanand Lanzhou will have an average temperature of 25 C. So you can take along short-sleeved and long-sleeved shirts as well as the light coat to keep warm at nights.
The average temperature of Dunhuang could be about 28 C. Sometimes, the highest temperature of Dunhuang can reach to 34 C in daytime. But it will fall a lot at nights. So apart from the summer clothing, you'd better prepare some light clothes for youe kids.
As for the weather of Lanzhou and Xian, the latest weather reports showed that the average temperature could be about 25 C or a little higher. Meanwhile, the sudden thunderstorm may happen and pass off quickly.