The Pact of the Isle of Faces
After centuries of killing, the two peoples met on an island in the middle of a lake and agreed to share the country: the open land to men, the deep woods to the children, and every weirwood given a face so the old gods could watch the bargain hold. The Age of Heroes begins here ÔÇö four thousand years of legend the maesters cannot verify and the northmen will not doubt.
The fuller tale
After some two thousand years of a war neither side could finish, the two peoples met on an island in the middle of a great lake at the centre of the country and made a bargain. Men would keep the open land ÔÇö the fields, the coasts, the river valleys ÔÇö and the children would keep the deep woods, and neither would take from the other. Every weirwood on the island was given a face so that the old gods, who were the children's gods, could witness it.
That the meeting happened on an island is worth noticing. Neither party could be ambushed there and neither could bring an army, which suggests two exhausted peoples who had learned exactly how much to trust each other. The order of green men said to keep the island afterward is either a real priesthood or the way a later age explained why nobody farms good land in the middle of a lake.
What follows is called the Age of Heroes, and it lasts four thousand years, which should tell you how little of it is history. It is the age of Bran the Builder, Lann the Clever, Garth Greenhand, Durran Godsgrief, the Grey King and Symeon Star-Eyes: names that every great house in Westeros descends from and that no maester can date within a millennium. The Citadel's private view is that most of them are titles worn by a dozen men each, or by nobody.
A peace that held, and then did not
The Pact was kept for a long time by the standards of anything else in this chronicle ÔÇö long enough that men and children fought side by side when something worse came out of the north. But the terms favoured whoever was growing, and it was men who were growing. The deep woods shrank a field at a time, without any treaty being broken in a way a maester could point at, which is how most agreements between an expanding people and a shrinking one end.
The chronicle's map keeps the children's woods painted after the Pact for exactly this reason. Show the continent as one colour the moment the bargain is struck and you have told the reader the opposite of what the bargain said.
Elsewhere in the wiki
 See the world as it stood in 10,000 BC What became of it  if an ending is written, it is here. Spoilers.
The chronicle records no ending here ÔÇö or none the maesters have set down.