When it comes to buying apparel sight unseen, customers generally rely on manufacturer description of sizing. Even then, often times manufacturer sizing guidelines can be inaccurate. That's why the vast majority of online sellers of apparel offer free return shipping. Not Little Sleepies.
First, their sizing guidelines are hilariously wrong. We bought a pajama set for our daughter based on their sizing description. What we received was a pajama shirt that fits as expected but pants that are at least three sizes up. Even worse, when we initiated a return request, we were presented with options of either store credit for free returns or self-paid $5.95 returns label.
In other words, once they got your money, even if their products don't fit the way they describe, you still get penalized just to get your money back. Retailers like Little Sleepies should be ashamed.
TL;DR: Little Sleepies have bad sizing and costly returns even for NWT product returns.