I have bought quite a few Crocs for kids over the years. I tend to size up and it's never been an issue until now.
These Crocs look identical to the photo, color, style, secure strap slightly shorter than the heel. I was buying fro a size 4(Toddler) shoe and purchased a size 4-5 Croc accordingly. The width was great for a pudgy little growing foot of this size, however, the length was just right WITHOUT the rear strap. It I used the heel strap the foot was packed into the toe of the Croc and I could barely stretch the strap back far enough to hook over the heel. I could try to stretch out the strap over time, or when not in use, but the main point of give is the strap hinge/joint/pivot(where's it's attached to the Croc) not the strap length itself, so I wouldn't be gaining what I need.
I did have a pair of Crocs in 6-7  in the 'Crocskin Classic K' and it was huge compared to this. I'd say it was about the size I'd expect, but these 'Crocband II' in size 4-5 was better suited for a size 4 or smaller in my case, nobody over a 4Toddler.
Same Croc standard, and quality otherwise, and as attractive as the photo. It was even the true inflated price of the Crocs line, just the sizing that was deflated...