Wellington winger George Bohn scored in the corner on a kick regather, and then reserve prop Ta-Ori Lupo was gifted a try when Wanganui spilled the ball 10m from their line.
It could have been more if a few of the teenagers were not penalised for backchat by referee Mark Wilson.
Turning with the breeze, Wanganui gave themselves some hope when winger Sailosi Naqiso crashed over under the posts shortly after the restart for 19-7.
But Wellington switched to off-the-ruck tactics and eventually their big lock James Blackwell took a quick tap at the line and forced his way over.
Set pieces fell apart in the final quarter as both teams opened it up for some more helter-skelter play.
But Wellington counterattacked through the midfield every time as the tackles fell away.
Winger Salio Siaunua crossed in the corner, and then reserve forward Jack Ross finished off another breakout on fulltime.