Dec Weather, scenery in jiuzhaigou & kunming

In Dec. it is cold in JZG and it may snow a little. Also taking its far-away location and common scenery, I don't recommend you to go there. While, Kunming and Lijiang in Yunnan Province are better choices. It is warmer there and there will be many sunny days. The highest temp may reach 15-18C and the lowest can be around 0-3C. You should still prepare warm clothing for kids and the elderly.

Thankyou very much for your advice & suggestion, Edgar. Btw, what abt Henan, Xian?

Well, Henan is a big province in China. Take Zhengzhou as an example, the temperature may range from 30 F to 50 F. It will be cold.
For Xian, it will be 30 F to 40 F. It will be colder than Zhengzhou. Maybe you can see the beautiful snow-covered landscape. In addition, you need to prepare the winter clothes such as down jackets, sweaters, snow boots, pants, scarves and gloves.
Generally, december is the winter in most part of China. If possible, you can make your trip in the warm time. Do your kid have summer holiday? It will be a good choice.

Thanks for your prompt reply & suggestion, Jessica.