I went to Nanshan last X'mas. If your kid knows how to ski or planning to learn skiing, Nanshan is a good place. They have shuttle bus to depart 三元橋 and the travelling time is around 1 hour. I'll go there again in coming CNY.
You have to reserve the shuttle bus seat 1 day in advance. For the booking tel no., you can check it at Nanshan website. The bus departs 三元橋 at 8:30am every day.
I went to Science Centre with my daughter last X'mas, 1 day is really not enough. I'll go to Beijing again for skiing in coming CNY. If Science centre will open after 'Chor 4', I'll definitelyy go again.