How much is an annual pass of Ocean Park for child&adult? I hear there are different kind of passes.

You are right. The annual passes are classified to Children Pass and Adult Pass.
Adult Pass (Peoplpe who are more than 12 years old): Gold Pass, 780 HK$ and Silver Pass, 610 HK$.
Children Pass (Children between 3 and 11 years old): Gold Pass, 390 HK$ and Silver Pass, 305 HK$.