When your kid(s) asks, ... don't be ashamed to answer.
Here is one way to explain:
Do you know how, when you put plant seeds in the earth, and then after some time the fruit comes out (on the plant)?
Same thing.
First a bit of an explanation about sex:
When two people are in love, they like to do things together. Sex is one of these things.
During sex, men puts his penis in woman's vagina. If feels good, so some people like to do it often.
It's like dancing, only more intense.
During sex men puts seed inside of women. Seed is in a sort of a liquid form.
Inside a woman's body, semen is joined with the second part needed to make a baby: an egg.
(Egg is produced by a woman's body.)
Baby grows inside a woman's uterus (like a pocket in kangaroo) for 9 months and then comes outside.
More info here:
http://www.babycentre.co.uk/preconception/activelytrying/howbabiesaremade/