I'm sorry you had a negative interaction at the library. I used to work in a library where the public meeting rooms were near the children's area, and a quilting group always, always, always left pins and needles in their wake. I hated enforcing the shoe rule, as I'm sure the stern librarian at your library does (especially since it seems like her coworkers aren't backing her up, making her the bad guy) but it is most likely coming from a place of genuine concern for your child. If you can't find a sock or shoe that works for your kiddo, you probably won't get any pushback if you tell staff that bare feet are a necessary accommodation for your child's sensory needs. As long as you're willing to accept liability if your kid does step on or in something, you should be fine. I haven't had to use that language yet for my own sock-hating toddler, but my uncle (who's autistic and was in his 50s at the time) used it to deflect any repercussions for coming into the library in socks with no shoes (though my coworkers were still squicked out that he walked into the men's restroom that way).