By Ian McCullough
His twin brother George was also included in the starting line-up with older brother Sam joining the remainder of the Rabbitohs' first-grade squad on the sidelines.
Back-rower Ben Lowe opened the scoring for the hosts, crashing over from close range after nine minutes, but the Kumuls' response was immediate when halfback Israel Eliab got his fingertips to Dion Aiye's grubber-kick for a try.
Souths continued to press, but the tough-tackling PNG side kept them at bay before taking a shock lead on the stroke of quarter-time.
Eliab, who has been watched by a number of NRL clubs, picked off an erratic Souths' offload and raced 80 metres to put the ball under the posts, then added the extras.
The second period followed a similar pattern with the Rabbitohs seeing a lot of the ball and the Kumuls working hard to defend their goalline.
But that resistance was broken when Dave Tyrell darted over for his side's second score, with Goodwin kicking the conversion and, just before halftime, skipper Jason Clark restored Souths' lead from close range.
As the combative Kumuls tired, tries from Chris McQueen and Aaron Gray iced the game for Maguire's side, before Bennett Leslie and Clark added some extra gloss to the scoreline.
Goodwin finished with four goals for the game and Apisai Koroisau was also successful with a kick in front of the posts.