I don't have any research as I don't do much in the children's marketplace, I mostly work with business customers. One of my clients has a children's book, which I did a kindle and epub conversion for, but I wasn't involved in the writing of the book at all. For me personally, I tend to write things as simply as I can and still adequately explain my topic in order to make it as accessible as possible to as many people as possible. There are a lot of adults that aren't very strong readers, and it's always best to be as accessible as possible.
That said, if you want to see how advanced your book is under the Gunning Fog or Flesch-Kincade readability scales, you can turn them on directly inside Microsoft Word and have it report the statistics to you. Here's the directions for how to do that: http://office.microsoft.com/en-us/word-help/display-readability-statistics-HP005189601.aspx?redir=0