There's no reason why children's fiction shouldn't be properly researched. In fact feeding children poorly thought through, shoddy nonsense might even be construed as a form of child abuse.

As a consumer of fiction I prefer it if the authors I read know more than me about the subject/subjects that they are writing about. And if it's fantasy that I'm consuming I need to be able to suspend my disbelief. Therefore the particular fantasy world needs to be consistent and properly thought through. I hate the type of fantasy in which the author has contempt for the medium (and his/her audience?) and thinks that he/she can make it up as he/she goes along without any regard for credibility or consistency. I suspect that most intelligent children would agree with me.