The women's eight finished the six-boat final in 6m 45.81s, less than 0.3s behind the Russian crew in third place, while USA were comfortable winners (6m 36.9s), followed by Great Britain.
Weenink said the two youngsters would have had little eights experience for linking up with the New Zealand crew, but given a Kiwi eight entered the Olympics for the first time this year, there was a lot of opportunity looking forward to Tokyo 2020.
The boat, with Union Boat Club's Rebecca Scown on stroke and containing Kerri Gowler, were also fourth in Rio - just like the Under 23's race they started fourth and stayed there, well ahead of fifth and sixth but right behind the medallists.
"It's an Under 23 class so you can see they're going to carry on with the women's eight at the Olympics," said Weenink.
"It just takes time in the system to get up there. They're on a good programme.
"They've gone from junior, then the next year they're Under 23 - it just takes a few years [to develop]."