We bought some Junie B. Jones for my 7 year-old. I was so horrified by the grammar/sentence structure that I found myself changing the words as I read. Some friends advised that my son would probably notice that she does not speak the same way that we do and ask about it, but we did not get very far, because he told me, after we were half way through the second one, that he doesn't like the books. Phew! We have quickly moved on to Judy Moody, same genre but MUCH more readable.
I read all 27 books to my daughter. We loved them all. I would read a few chapters to her each night. We started reading them when she was 3 and finished a few months after her 4th birthday. My husband is now re-reading the series to her and he loves Junie B. just as much as me. It is amazing how much our daughter has learned from the Junie B. books. It's worth the read.
My son read some of the Junie B. Jones books and asked me if the author was from another country because she didn't write in English well. I told him she was trying to write like she thinks Junie B. would speak. He said she must think Junie B. is not to smart. Readers get grammar. He's six and his verdict is that these books are funny. Give kids credit, they get it.