When my daughter was 10, I read parts of a puberty book to her. The best one is from American girl Library, "The care and keeping of you" You can get it used on Amazon.com or any mayor bookseller. The book takes about everything from personal grooming to body changes,period, pimples, emotions, everything and in a easy non-complicated language.
My daughter is now 12 and just got her period for the first time, I know that book prepared her for that very well.
Good luck!
I am a stay at home mom and have been married for 15 years. I have a 12yr old girl, 11yr old boy, and 5yr old boy.