we will arrive at beijing terminal 2 at 2am. what is the best way to get to novotel Xinqiao?

Since there are child, so it is suggested to book a pick up service at the airport. I used the pick-up service of TravelChinaGuide and the fare is around 660RMB. They can take you to reach the hotel directly. Email address is bookingtravelchinaguide.com.
The cheap way also exists. You can take the airport shuttle bus line 2 (Night Line) to reach local Railway Station and then take a taxi to reach your hotel.

Do u mean the airport shuttle run around the clocks or 24 hours?

No, it does not operate around the clock. Line 2 has the Daytime operation and Night operation and the routes are a little different. The night line will operate from 24:00 till the arrival of the last flight. Hope it is clear now!