I am a mother of two beautiful girls, 16 & 12. Let me say very clearly that I am no psychologist and I really wish I had had time for all of the "how to" books, but with little ones, I barely had time to shave my legs! lol!! I'm sure all you busy moms can agree with me!
I can say, though, that I have actually successfully dealt with that question...twice. My husband, like yours, thinks that the "sex education" needs to be on my list of things to take care of because he refuses to deal with that subject.
When my oldest came to me, she was almost 4 years old. I was pregnant with my youngest daughter. I just responded as carefully as I could (trying not to say too much and incourage more questions, or at least more uncomfortable questions...with answers she was too young to hear). I responded by saying: "Well, God helped me and daddy plant a seed to help the baby grow in my tummy." The next question from her was..."How did God help you and daddy get the seed in your tummy?" Okay, some people might disagree with my response but I was trying to keep it simple. My response: "We prayed really hard and eventually the seed was in my tummy and your little sister is growing now!" I realize that my response was a little vague; however, I didn't lie...we did pray for her. I also realize that you aren't dealing with a 4 year old child. If I was dealing with an 8 year old, I think I would explain it by saying something like: "well, your dad and I had to plant something like a seed inside me" of course, his/her next question will probably be..."How?" I would then just try saying "well, by doing something that only mommy's and daddy's do....or only what married people should do" (Whatever you feel comfortable with saying) You will just have to go from there with whatever you feel comfortable with. I mean, if you feel comfortable using the "sex" word, you could always bring it up (not necessarily EXPLAIN it), but I wouldn't think you should be scared to say the word. I can say from experience that the more you act uncomfortable, the more they seem to pick up on that and the more curious they seem to get...and more questions come! And so many times, they have already heard that word...they just don't necessarily know what it means.
I tried to be as casual about it as possible so it didn't "peak" more interest. I hope that makes scense. If you think about it, the stranger someone acts when they are telling you something...the more you pay attention and the more interested you become in what they are saying...same thing applies with a child.
I hope this helps. It might sound a little crazy to some...but it worked for me. I was worried that by saying too much, I was going to get a call from the Pre-school teacher asking me what in the world I told my child...or, a call from one of her friend's mothers asking me what I was thinking. Just keep in mind that they are going to get educated somewhere, it might as well be from you...where you can at least control what is being said!
Good luck and let me know how it goes! :)