The Keepers Legend (unedited)

It is believed that the keepers did have children. Princess Celestia herself believes that there is a reason the main six became the elements of harmony: because they are the descendants of the keepers.

Princess Fauna is the ancestor of the element kindness, Fluttershy.

Prince Typhoon rage (the keeper of wind) is the ancestor of the element of loyalty, Rainbow Dash.

Princess Ember, the element of fire, is the ancestor of the element of honesty, Applejack.

StarSwirl the bearded (the keeper of magic) is the ancestor of the element of magic, Twilight Sparkle.

Princess Oceana, the keeper of the element water, is the ancestor of the element of generousity, Rarity.

Prince Felix, the element of emotion, is the ancestor of the element laughter, Pinkie Pie.

Celestia and Luna never had children so they stayed the gaurdians of the sun and the moon, but who knows maybe love will find them someday.

It has not been completely confirmed but princess Celestia believes it to be the true reason why the main six became the elements if harmony.