I suggest the 3 days Top of Europe Winter Pass for the Jungfrau area.
For the rest of the days, I do not know yet as all depends on the length of your stay. I would have said, the Swiss Half Fare Card including complementary Swiss Family Card. But if you only stay in Zurich, the Swiss Half Fare Card is not worth either.
I wondered why you want returning to Zurich for New Year? Family reasons? If not, there is no point in going to Zurich. New Year is not really a big event in Switzerland, especially in cities, apart from the fireworks display in Zurich after midnight; shops and attractions will be closed from 31 DEC late afternoon including 01 JAN / 02 JAN. You would better spend the New Year in the mountains, there is where people spend New Year. Your 8-year will be bored to death in Zurich while I do imagine he/she will absolutely love sledding and playing in the snow. Snow in the cities is very unlikely to happen.
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