First, I’d be much more likely to take them to a library than a bookstore. I spent a lot of time in the public library – I inhaled books, my mother a’h used to say. So too did most of the yeshivishe and frum velt in my neighborhood. at the time, it was not only considered appropriate, it was encouraged. Not too many people had televisions, and of the homes that did, responsible parents didn’t let their kids watch much at all. Movies were a rarity, and kids played outside. (I’m not talking about the 40,s, I’m talking about the 70’s).
I’m certain that the quality of some of the books in the library has declined, in both appropriateness and simple style, but the vast majority of materials available for kids, taken by a parent who can scan books before they are taken out, is still quite good.
Bookstores, especially the big chains like B&N, cater to a broader market, and carry what sells. If inappropriate sells, they will carry it. So I’d be very careful, and maybe go if you need to without the kids.
HOWEVER, let’s not make the assumption that every thing that is not heilige is evil, as some posters here have explicitly stated. There is a huge amount of knowledge in the world that is neither pure nor evil. Is there something evil about calculus? trigonometry? chemistry? instrumental classical music? landscape paintings? a children’s story about a big red dog? Of course not.
We do not live in a dualistic world, like the Zoroastrians or the Gnostics believe. We live within the creation of the Aibishter, who is the source of everything, and he gave us the knowledge to distinguish between the heilige, the evil, and everything in between that we have to use our seichel, our judgement, and our Torah education to assess and determine its worthiness.