Dinwiddie (2-10 from five overs) and Toby Lennox (2-20 from seven) were also extremely effective.
The Wanganui U13 team reached their target of 156 with five wickets in hand and plenty of overs to spare.
Earlier, every player bar the wicketkeeper got a bowl, with Hadleigh O'Leary making the biggest contribution, 2-24 from his seven overs.
Harry Godfrey took 2-11 from his four overs, while Joel Clark had 2-18 from his four.
In reply, Wanganui took 26 overs, 10 fewer than their Horowhenua Kapiti opposition, to reach 158, helped by Adam Lennox retiring at the top of the order after scoring 55 from 43 balls, including eight boundaries.
His opening partner Godrey added 24 and while Wanganui had some concern with run-outs and went from 116-3 to 130-5, they still snaffled the remaining runs in short order.