In the ancient texts of Hyrule, the legend that the fate of Hyrule would be determined on the night of the Queen's death. It never told why, just that it would be determined. Many people believe that on the night of the Queen's death, the King would be so overwrought with grief that he would take his own life, leaving the kingdom of Hyrule to their young daughter. The truth of those words is the fact that the night of the Queen's death, was the night the Hero of Time was born. A great evil was foretold to sweep over the land, and that the child would be able to stop it, with the assistance of seven others, not of the Sages. No one knew which child born that night was the Hero of Time, thus any and all children that were found were slain.
But other villages had come under attack as well as Hyrule Castle. The Legend speaks of the seven other children, each born as an embodiment of their representative temple's powers. Fire to fire, forest to forest, water to water, and so on. These children were gathered by the Sages, sacrificing their memories as a Sage to send away the seven children to live in peace, in another world, to grow up until their time of Destiny arrived. No Sage knew then, where the children went, or have any recollection of their power as a Sage of their temple, to protect the children.
Why were these children so important? They are the Guardians of their own respective temples, the embodiment of the power of their temple. If they were to come under the power of the great evil, the ridding of the evil would become almost impossible. The Gerudo king, Ganondorf, knew this, and thus attempted to seal the fate of Hyrule under his rule by killing the children and the Hero of Time. When the Guardian's time of Destiny is at hand, they will be brought back to Hyrule, still under the protective spell of the Sages until the time comes where it is broken. Each will be recognized by the color of their tunic: Green, Forest; Red, Fire; Blue, Water; Purple, Shadow; Orange, Spirit; Yellow, Light; and White, Time. Texts say that once the seal is broken, the Guardians will become stronger, but their appearances will change as well, back to their birth.
The journey is long, hard, and grueling, is what the Legend says. May the Goddesses bless them, and watch them, guide them through their journey and have mercy on them, for they are the only ones that can save us from this great evil.