What must I do if I suspect a child at the club has been sexually abused?

Report it to the police immediately. Under section 54 of the Criminal Law (Sexual Offences and Related Matters) Amendment Act 32 of 2007:

• Anyone who has knowledge, a reasonable belief or a suspicion that a sexual offence has been committed against a child (or another vulnerable person) must report it immediately to a police official.
• Failing to report is a criminal offence, with a fine or up to five years' imprisonment, or both.
• A person who reports a reasonable belief or suspicion in good faith cannot be sued or prosecuted for making the report.

The duty is personal: telling the club's safeguarding officer or the federation does not replace reporting to the police. You can also tell your club and federation safeguarding officers so they can follow their own policy.