In an interview a couple years ago Aonuma mentioned something on the Koroks being the forms Kokiri take when leaving he forest
"We created the Rito as the evolved form of the Zora that appeared in "Ocarina of Time" and the Korogs as what the Kokiri became once they left the forest. They appear different, but they have inherited their blood." —Eiji Aonuma
It could be possible that if they leave the forest for too long they "loose themselves" or stop being a Kokiri and take on a strange wooden form. Then they will not be a Kokiri but something else, the Kokiri that wondered out from the forest is essentially dead and they are now this Korok. Why the transformation happens might have something to do with the forest being the Deku Tree's domain and while anything is within it, they are inside his power. But outside of the forest they are away from his influence, or if they are just away from it for too long. Since they are the creations of the Deku TRee it makes sense that their "real" form is something more wooden and related to the forest, and that the Hylian child look is more of an enchantment given to them.
"Oh, you have a fairy now?! That's great, Link! What? You've been called by the Great Deku Tree? What an honor! He may give you a special gift! Tee hee! That's because the Great Deku Tree is our father, the forest guardian, and he gave life to all of us Kokiri!" — Kokiri
While the Koroks are called the spirits of the forest on a few occasions.
"Well, then, Link... These are the Koroks -- the spirits of the forest. [...] Now they fear people...but to me, they will ever be my cherished little children." — Great Deku Tree
The Koroks spend most of the year away from the forest haven, only to return for one day for their celebration. But the Kokiri only leave for the Lon Lon Ranch for a night, so there might be no problems for them as long as they return quickly enough.