Beijing-Xian-Shanghai in 8 days, 1-9 January 2015? Possible to be done?

Hello,
I am a family of 4 (2 children, 13yr and 7yr), will visit BJ on 1-9 Jan 2015. Have flight tickets, thinking of returning from Shanghai. Thinking of going to 3 places in 8 days, by train (softsleepers). Will it be possible? (we never experience Winter!) And is it possible to enjoy those 3 cities as well in Winter? Please advise other itinerary if not possible