Do I need consent to post photos of junior players on the club's website or social media?

In almost all cases, yes: the prior consent of a parent or guardian.

Section 34 of POPIA prohibits processing personal information of a child (anyone under 18). Section 35(1) lists the only general exceptions. Processing is allowed if it is:

1. carried out with the prior consent of a competent person;
2. necessary to establish, exercise or defend a right or obligation in law;
3. necessary to comply with an obligation of international public law;
4. for historical, statistical or research purposes that serve a public interest (or where asking for consent is impossible or disproportionate), with sufficient guarantees that the child's privacy is not disproportionately affected; or
5. information the child has deliberately made public with the consent of a competent person.

Posting match photos or highlights on a club website or social media is publishing ("dissemination") and counts as processing (s1). Of the exceptions above, only prior consent of a competent person will normally fit. Without it, the club would need the Information Regulator's authorisation under section 35(2).

Source: POPIA (Act 4 of 2013) ss 1, 34-35.