If one player in my child's team tests positive, does the whole team lose the match?

Not automatically. Under Article 11.2 of the SAIDS Anti-Doping Rules, the sanction falls on the individual player who committed the violation.

The team itself is sanctioned (for example loss of points, or disqualification from the competition or event) only if more than two members of the team commit an anti-doping rule violation during the same event period. The event's ruling body may set stricter team rules for its own event (Article 11.3).