I picked up a pair of these shorts (in khaki) for my kid in a size 12, with the hope that she could wear them at the end of this school year & the beginning of the next. I don't think that's going to happen. She has been wearing size 10 (or size L/Large10-12 if the company doesn't carry single-number sizes) for quite a few months, and I've even had to sew a quick dart or two in the waistband of some bottoms to get a better fit for her.
THESE shorts, however, are fairly stiff. Even though they are pull-on shorts & have a nice comfy stretchy waistband (which is great for her mild SPD), they don't really have any stretch OTHER than that. It's almost as though they took a pair of shorts that would normally have a fly front & decided to put the covered elastic waistband on instead. She has some trouble getting them pulled on over her hips, though once they ARE in place, they're comfy and wearable. As long as she remembers to stand still & keep her legs fairly close together, pulling them up is...doable. Of course, this can be frustrating for a kid impatient to get back to class or recess or whatever.
With the elastic waistband, I definitely expected them to be a LOT softer & stretchier, something more akin to the Old Navy khaki jeggings. Sadly, it's like they took a pair of their typical khaki twill shorts & slapped an elastic waistband on them.
I would size up, if not just find something else. As I said, once she has them on, they are comfortable & the waistband is great & doesn't dig into her navel (like fly front shorts often do) and is nice & thick. BUT it takes some work to get them on, which frustrates. That said, she DOES still reach for them herself, so at least my 9 year old doesn't find it SO frustrating that she deems them unwearable, so take that however you will.
-BML