Singaporeans are going to visit China and Hong Kong. What visas do we need?

My family (2 adults & 1 child, holding Singapore Passport) is planning a Free & Easy 9-Days holiday to Guangzhou & Hong Kong coming Dec. (SG --> GZ --> HK --> SG)
If I understand correctly, we are exempted? If not, do we apply VISA for Single entry of 3-month validity?
What do we need to prepare when entering HK via train from GZ?
Thank you.