Check out the Usborne selection, they have several depending what you're after. They are easy to read and have pictures.
Babies and Growing Up are printed separately at £5.99 each or Understanding Facts Of Life is a combined volume at £8.99. These are aimed at children aged 8+, there are others, You and Your Body, Flip Flap Body book aimed at age 4+. there is also an internet linked one, Human Body but i think Facts of Life would probably be more appropriate.