Ideas
This is really unbelievable. I checked 6 or 7 of the two dozen children in my save, and all of them are affected.
I always take a screenshot in CAS when I'm done creating a character, so now I can compare them pretty well before / after today's update.
Check this out (before on the left, after on the right):
To make sure the boy in the third image did not just get fatter over time, I loaded an old save file from just after his creation to create the "after" image. So actually, both images were made shortly after the boy's creation, the left one with the Sims version on January 17, the right one with today's version.
I included mostly boys because the effects are much more prominent on them than on girls (for my Sims).
Your child Sims in the first two pictures look like the skinny, sort of stretched child Sims I checked from my Library.