12 days family budget traveling start from Tianjin-Beijing-Nanjing-Wuxi/Suzhou-Shanghai-Hangzhou

Yes, it is OK for 2 adult and 3 kids. It is cold during Nov and December in those cities, especially in Tianjin and Beijing. You need to wear long-sleeved T-shorts, Jacket and jeans, better take wool sweater and overcoat. There are bullet/high speed trains among those cities and it is convenient for you to go around by train. You can find hotel without early booking, but I think book the hotels in advance would be much convenient for you when get there.