Is it suitable for children traveling in harbin on December? Can the weather be dangerous for them?

It will be fine for children. Although it is cold in December, it is not that dangerous. There are heat systems indoors, and when going for outdoor activities, just wear heavy coat, down jacket and boots to keep warm. I had been there once when I was a kid, there was no big problem. Actually, I really enjoy the ice and snow world. It is unnecessary to worry about that, just make full preparation for it.