How do we get to Millenium Harbourview Hotel from Xiamen airport late at night?

Since you have 7 adults and 2 children, I think the pick up service is the best choice at night.
Or you can take the airport shuttle bus to reach local railway station and then change to take taxi to reach your hotel. But you may need at least two taxis.
Or you can contact TravelChinaGuide and they may provide you the pick up service, but I am not very clear about its price. You can ask them by sending them email emails at travelchinaguide.com|booking for asking.