I'm saddened to learn of thrift stores just tossing their pre-1985 children's books in the dumpsters. I understand that by nature thrift stores can't claim they are selling collectors items (since they are a "thrift" store, not an antique shop). However, they shouldn't be dumped. I would think they could be donated to a college or university library. By definition these libraries are for adults, and these books could be used for research purposes and preservation of history--which is what Universities do.
Arguably, a high school could also take these books. Again, by definition a high school library is for high school students--and it's unlikely that any younger children would have access to these books there. Where I did my student teaching they had classes on writing children's literature, and we did use children's books as examples in class and for students to use for research. Also, there were some high school students with disabilities that were actually reading on that level, and these books would be valuable to them. Picture books could be donated to high school art rooms to be used in collages...turning them into wall art which is also exempt.