Ideas
[Edited as this is reproducible without mods.]
Deleted the Mods folder before starting the game for the first time after applying the (25th Feb) patch.
I looked at child sims in various saves: an old save from HSY times (no mods ever touched it), a save that had seen mods (with mods disabled) and a brand-new save file created after the patch. All had noticeably deformed child sims. I noticed the unusually buff shoulders, inflated belly or disproportionate behinds. I can't really tell if they got skinny or not.
After identifying pre-made child sims that appear deformed in one new save, I created another new save and checked the same child sims. They have the same deformity. The child sims are Christie Sigworth (Strangerville), Maira Watson (HenfordOB), Olive Tinker (EvergreenH), Orange Bailey-Moon(DelSolV), Wren Grove (ChestnutR)