Baby’s butt?  Really?  I don’t know who coined that phrase, but i don’t think they changed many diapers.  Guess where babies get diaper rash?  Rash isn’t soft.  Guess where baby waste goes?  The diaper doesn’t absorb it all.
Add to this mix elastic band impressions, chubby folds of skin that are hard to clean, and not getting a lot of air…  Soft isn’t the first word i would use.

But there are many parts of a baby that are indeed soft.  I’m particular to Micah’s calves myself.   Maybe the bit of belly off to the side, beneath the ribs where love-handles may one day form.

I guess “Soft as a babies lower leg” just doesn’t have the same ring (and daring-do?) to it.  So don’t be surprised if something described by this inacurism actually turns out to be… sticky?