How to get to ocean park from marriott sky city hotel in Lantau, Hong Kong?

You can take airport express at Asiaworld Station, get off at Kowloon Station first.
Then take bus no.973 at Western Harbour Crossing Toll Plaza, and go to Wong Chuk Hang San Wai, Wong Chuk Hang Road. After arriving, walk southwest around 300 yards to find the scenic spot.
The wholeway takes around 85 mins. And the ticket fare is about 104 HKD per person.

How much would it cost if by taxi? We are a family of 6. Husband and I with are 4 children (3small and 1 big)

If taking a taxi, the fare could be HKD300 for a cab, but you six may cannot share just one taxi, so two cabs or a minivan are suitable for you.