The way to restrict them from inappropriate material is to go with them and look at what they are reaching for. Or go with them prepared with a list of books that you either prepare yourself or you have asked the librarian to assist you with.
There are many great authors and many great books that WE read as kids that can still be found in libraries today, which are perfectly legitimate for our children and grandchildren. However, I would not allow them the freedom to roam around the library by themselves until they were old enough to understand the dangers of what they can fall into including innocent children’s books discussing alternative family styles like two mommies or two daddies. Or how we celebrate with a Chanuka Bush, or why our Passover Seder is different than our neighbor’s.
There are many biographies on very important people including the people who founded this country and Eretz Yisroel. There is a vast amount of knowledge to be found at the library but just as Shoprite and Walmart have both kosher and non-kosher so does the secular library and until kids are old enough to know how to look for the proper hechsher on books, they should not be left alone to pick and choose.