Your daughter is trying to identify with the same gender parent as she is probably beginning to notice that there are differences between genders. She is also beginning to understand there are different relationships between people. Although some may think that nine is still young, it is actually pre-puberty and the beginning of this interesting phase in her life. Marketing strategists target this age group by appealing to them as though they are already teenagers and stress a more grown-up physical appearance.
It can be awkward, sometimes painful, to gracefully move out of childhood into adolescence. Appearance, acne, menstruation, etc., need to be addressed so that the young lady develops a healthy self-esteem at an awkward time in her life. The fifth grade is usually the time when many schools present sex education to their students. When public schools teach sex education, the main focus is usually on the biological changes and the hygienic rules. While this information is important, it presents a very shallow understanding of human sexuality. As Christians, we need to emphasize the spiritual dynamics and the commitment to purity and chastity without coming across as rigid and closed-minded.
Answer your daughter's questions sincerely and appropriately according to her age. Focus on the spiritual and emotional aspects without neglecting the physical, including the changes that she will begin to develop or notice her friends developing. Your description does not need to be totally graphic at all, but respectful, so that she comes away with a wholesome understanding that all these differences work together in a beautiful way according to God's design and plan. Lead her to understand the wonder of the Sacrament of Holy Matrimony and when it is appropriate to express physical affection towards a spouse. Gracefully, assure her that you will always be available for her if she has any questions and that you will be as honest with her as you can.
There are many books that address how to discuss sexuality with children and youth of all ages from a Christian perspective. However, you do not necessarily need to use a book to explain what you want to say; just let your words come from the heart. Even if you struggle with exactly what to say, it will help you and your daughter together to appreciate this sensitive topic with the reverence it deserves.
The book, "Sex Education," by the Rev. Father Guirguis Guirguis is a very good general resource to use for any age.