I try to do honest reviews, but never hurtful. Most of the time if a book doesn't appeal to me, I will just stop reading it and not do a review. It is a rare occasion that I feel the need to talk about a book I consider no bueno.
I have no rating system, but I do an "IOS" (In One Sentence) and a "Bottom Line". It is just too limiting for me to do a stared review. I feel that every book is unique and should be treated that way. Plus, everyone has a different idea of what constitutes a four-star, three-star, etc...
While I do read and review childrens', middle-grade, young adult, and comic books, I can not be the one to say if a book is okay for your child to read. I might point out when a graphic novel is a little too graphic or that a young adult author is not afraid to tackle hard-hitting topics like drugs or alcohol, but always remember: Parenting styles differ across the board. What one parent would find acceptable, another might think vulgar.
So, the best advice I can give you, if you are concerned with your child's reading habits, is: read the book in question and start a conversation with your child!
One of the greatest things my father passed on to me was a love of reading. It is a trait that I treasure and I'm very lucky that every time I finish a book, I think of him. So, maybe, if you discuss reading with your child, you'll not only be aware of the ideas they're exploring, but you might start something that they will carry with them for the rest of their lives.
Plus, you might just enjoy some of these books yourself. It's a WIN/WIN situation!