We are 2 adult + 1 child, plan to visit Tung Chung from Causeway Bay and that cable to giant buddha

It is suggested to use the local mtr to reach there directly.
From Causeway Bay Station, you can take the Island Line to Central Station. Then you can get out of the station and try to find the HK Station nearby. From HK Station, you can take the Tung Chung Line to reach Tung Chung Station directly.
The fare is around 20.2HK$ if using the Octopus Card per adult and around 10.1HK$ per kid.