The Harold Matthews under 16s scored a huge 42-6 walloping of the tricolours, with the SG Ball under 18s registering a 30 points to 14 victory.
The Harold Matthews side led 18-nil at the half time break before smashing their neighbours in the second stanza.
Gabriel Hamlin, Nat Butcher, Reimis Smith, Siosifa Talakai and John Neil were best for the red and greens.
In the SG Ball game, the Rabbitohs trailed 14-12 at half time, swamping their opponents in the second half to keep them scoreless.
Paul Momirovski and Cheyne Whitelaw were the standouts for South Sydney.
South Sydney will move on to play Balmain at Redfern Oval in round five of the junior reps competition.
South Sydney 42 (Gabriel Hamlin 2, Jarrod Cullen, James Ferguson, Jack Hawkins, John Neil, Reimis Smith tries; Siosifa Talakai 7 goals)
Sydney Roosters 6
South Sydney 30 (Paul Momirovski, Liam Coleman, Hayden Preece, Karl Saena, Cheyne Whitelaw tries; Paul Momirovski 5 goals)
Sydney Roosters 14