Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="parent">
  <h4 class="title">Title</h4>
  <div class="child">Child 1</div>
  <div class="child">Child 2</div>
  <div class="child">Child 3</div>
  <div class="child">Child 4</div>
  <div class="child">Child 5</div>
  <div class="child">Child 6</div>
</div>

SassMeister-input.scss

// ----
// libsass (v3.2.5)
// ----

@import "bourbon/bourbon";
@import "neat/neat";

.parent {
  @include outer-container;
  border: 1px solid blue;
  padding: 1em 1em 0;
}
.child {
  @include span-columns(6);
  @include omega("2n + 1");
  border: 1px solid red;
  margin-bottom: 1em;
  padding: 1em;
  text-align: center;
}

SassMeister-output.css

html {
  box-sizing: border-box;
}

*,
*::after,
*::before {
  box-sizing: inherit;
}

.parent {
  max-width: 68em;
  margin-left: auto;
  margin-right: auto;
  border: 1px solid blue;
  padding: 1em 1em 0;
}

.parent::after {
  clear: both;
  content: "";
  display: table;
}

.child {
  float: left;
  display: block;
  margin-right: 2.35765%;
  width: 48.82117%;
  border: 1px solid red;
  margin-bottom: 1em;
  padding: 1em;
  text-align: center;
}

.child:last-child {
  margin-right: 0;
}

.child:nth-child(2n + 1) {
  margin-right: 0;
}

SassMeister-rendered.html

<div class="parent">
  <h4 class="title">Title</h4>
  <div class="child">Child 1</div>
  <div class="child">Child 2</div>
  <div class="child">Child 3</div>
  <div class="child">Child 4</div>
  <div class="child">Child 5</div>
  <div class="child">Child 6</div>
</div>

Exploring 2-columns x 3-rows using Sassmeister. Investigating which way the child elements loaded (LTR, top-row downward). Note: .child:nth-child(2n+1) is similar to neat.io's omega("2n+1") function.