Hi~
I'm 51 and 'been there and done that' (2 boys and 2 girls). Menstruation and sex are simply a part of life like eating, sleeping, talking, worshipping, reading, etc. Don't be nervous. People have been menstruating and having sex (and babies) since Adam and Eve and it's nothing new. God created it and it's 'all good'.
Be as matter-of-fact as you can without emotion -- and take advantage of every time THEY ask you a question, but only answer what they Actually ASKED w/very little elaboration. After you answer a few questions satisfactorily, they'll come to you with more later, don't worry! You WANT them to feel free and to trust you with their queries (vs school system or TV or their friends).
You might lead into it with your son while you're in the car with just him and the radio or a CD is playing (not to loudly, I hope! LOL Just make the scene as 'natural' as possible) and say something like, 'How would you explain where babies come from?' Let him talk til he stops w/o interrupting him. Depending on the accuracy of his response, ask him where he learned 'this' or 'that', correct any misconceptions, and ask him if he has any more questions (like, now). If he does, simply answer them; and be sure to tell him that he can come to you or his dad any time with questions about life -- that you want to be 'there' for him.
You didn't say how old your daughter is, but a friend of mine started her menstrual cycle at 9 (back in 1966), so be prepared at least that early! A little booklet with diagrams (to look at WITH her -- NOT to 'give to her') is helpful. You can point out where your and her ovaries are, the uterus, etc. Tell her that each month an egg is released, and if it's not fertilized, it goes into the uterus (which has prepared to incubate a baby 'just in case'), and then it all sheds away if not needed.
If you get your children coming to you about these important issues before puberty/adolescence, you'll have their trust for life.