Who made me?, Malcolm and Meryl Doney
This book was the first one we got and it has been a good one. It is suitable for the 5-9 year bracket, but could easily be simplified and read to younger children (who can’t read the bits you are skipping!). Illustrated by Nick Butterworth and Mick Inkpen, it has fun, simple illustrations, which even parents will like (especially the one of the parents sleeping in bed looking rather sated after making the baby!).
It is rather detailed in some parts about the act of intercourse, so you can adapt it to suit your reading audience, if you prefer.