Hi Baby4,
When my girls started asking, I just broke it down into simplistic terms.
I said that inside Mummy is an egg and inside Daddy is a tadpole The tadpole and the egg meet oneday and when they 'kiss' it starts a baby growing inside Mummy's tummy.
Of course, they were happy with that at four but as they have gotten older the questions have become more and more complicated. Now the twins are almost 11 and Lily almost 9, they know the whole story, so to speak. I think it is often better to be open and upfront with them, although, I am a midwife, so they have grown up knowing about pregnancy and birth and throw around words like posterior presentation and hyperemisis!! LOL. I guess you need to do what is comfortable for you.Good Luck!