Yours is a very good and thoughtful question.
We as parents are always teaching our children about the care of their bodies through the temperance and modesty that we choose in our own lifestyles. This witness will be the most powerful and lasting one they will remember.
In regards to the care of a young person's body, especially during the time they begin to reach puberty, it is particularly important that we as parents stress that the normal changes that occur in the body are a gift from the Lord.
We should give the necessary information about their bodies with straightforward answers that are neither too complex or too vague. Give only that information that seems appropriate for the question and the age when it is asked.
For example: When a 5 year old asks, "Where do babies come from?" this does not require a detailed discussion on reproduction. However, when a 11-12 year old girl begins to experience changes in her body, it is time to be more specific in our answers.
With regards to your question; when you begin to notice "breast budding" in your daughters (usually menstruation will start in 1-2 years) it is time to begin to educate them on the next phase of their maturity. Talk in a loving tone about menstruation and then gradually introduce reproduction. One topic naturally leads into another. However, it is not necessary to give all that information in one sitting. In fact it is better to wait until questions arise, allowing time for your daughter to digest what she has been told in small pieces. Believe me, they will think and ponder what has been discussed and have questions. Be open and ready to talk about this subject.
It is never too early to speak about chastity, purity, and modesty. Speak lovingly about the sanctity of the marriage and the gift of fertility. You will see that the subject indeed will come up when your children are around pregnant moms. This can be done as simply as saying a special prayer for moms that you know who are expecting. Remember that our children are always "listening" to our tone more so than our words. We should never speak with disdain about women who are pregnant outside of marriage. Instead we should openly pray for their babies and a healthy pregnancy. We should ask the Lord to continue to give these men and women (both sexes have a responsibility to safeguard pregnancy) the graces and courage necessary to carry a pregnancy to completeness during these days when the opposite opinion is held by the media and others. Your kind and loving tone will be a powerful witness to the difficulty and sorrow of pregnancy outside the Sacrament of marriage and the much greater sorrow of abortion.
There are two very lovely publications that I feel are not only sensitive to the information needed but are written with a beautifully Catholic ideology. They are Angel in the Waters by Regina Doman and The Joyful Mysteries of Life by Catherine Scherrer. I would suggest that you read them first, however, taking care to highlight sentences or passages that you feel would address the issues at present. Refer to these books as guidelines for your discussions with your little girl. When she is older she can read them on her own. The best explanations come from one's mom. This is the first 'woman' that a young girl is exposed to and naturally the most powerful of witnesses that speak of the beauty of our bodies.
Dearest Jesus, we love You and praise You for the gift of our sexuality. Help us each day to witness to the beauty of the human body and the spirituality of Your gift of fertility. Let us pray each day for the unborn babies who are in danger of abortion. Let us be compassionate and loving to those women and men who are faced with unplanned pregnancy knowing that all life from the preborn to the very elderly is precious and worthy. Amen.
Some day perhaps the Lord will give me the opportunity to tell you a couple of very funny stories from our family journal about teaching the facts of life and puberty. Until then my prayers are with you.
My husband and I (with 10 children), have had "pubertizing" young people in our family for many years. Some day they will all be grown and out of the house. I will miss them and the endless questions, though sometimes I suspect my husband is ready for a break.
Sending out a prayer,