it's a composite, and the clothes have been altered slightly to look their best.
at a guess they would have photographed the children in pants & vest/topless in a few poses in one session to superimpose lots of tops on.
the shadows and seams have small flaws when you zoom in. they've also made the right shoulder a bit too wide by mistake (to fit the cardigan on - probably from a mannekin? it's missing two darts which are clearly visible on the product in other pics. it's been brushed out.
artificial hands on most of her pics - and you can see here that her thighs are slender and normal - not missing a chunk off the side normally ;) www.hm.com/gb/product/06254?article=06254-A
also here they didn't get too happy with crease removals on the jeans: www.hm.com/gb/product/16265?article=16265-D
at a guess I think a much older woman with long very slim legs posed and when it's been matched up to the child's body it looks wrong because it's out of proportion with her frame - her "knee" has been made the same width as her upper arm!
same clothes, but better proportions! www.hm.com/gb/product/16265?article=16265-C
same child again www.hm.com/gb/product/00789?article=00789-K