My family (3 adults + two 8 yrs old child) will arrive Xianyang Airport Friday night about 11pm.

You can take the airport shuttle bus Longhai Hotel Line to Longhai Hotel directly. This shuttle bus will work to 02:00. The duration is around 70 minutes, and the ticket fare is CNY25/person. Upon arrival, you just need to walk about 370 meters to your hotel.
I have no idea what size of your big suitcase and the small ones, so I can’t make sure whether the taxi can hold them.