The bowlers justified captain Dasun Shanaka's decision to field as spinner Maheesh Theekshana struck twice in his first over after the skipper ran out Max O'Dowd for nought with a direct throw.
Hasaranga kept up the pressure with his twin strikes in his opening over and soon claimed skipper Pieter Seelaar.
Kumara returned to clean up the tail and claim T20 best figures of 3-7 with his pace bowling.
The Dutch though can take consolation that it was not the lowest total -- they were bowled out by the same opponents for 39 in a T20 World Cup match in 2014.
The Netherlands lost all their three games in the qualifying phase.
In reply, Sri Lanka lost two wickets before Kusal Perera, on 33, and Avishka Fernando, on two, steered the team home in 7.1 overs.
Sri Lanka join England, Australia, South Africa, West Indies and Bangladesh in Group 1 of the Super 12s.
Namibia are now clubbed with Afghanistan, Pakistan, India, New Zealand and Scotland in Group 2.