Either is acceptable.
Kids' is a genitive determiner. (Like our, the cat's, the man who stole my wallet's, and so on.)
Kids is a noun modifier. (Like machine in machine tools, book in book publisher and so on.)
The worst that could be said about the publisher is that it is being grammatically inconsistent. Mind you, for a book publisher, that's quite an indictment.
Aside: Often, when a noun is used as a noun modifier, it is forced into the singular form. For example, trousers is always plural, but the trouser in trouser press is singular. If this rule were strict, you could argue that the form using the noun modifier should be kid books rather than kids books, but exceptions like glasses case show that this rule is not strict. If somebody can explain why the rule is followed for trouser press and not glasses case then we may have a case against the publisher.