In Season 4 of Game of Thrones Bran met with a "child" who saved him and guided him to meet the Three-eyed-raven. What are those "children"? And are they different from the White Walkers?
The Children of the Forest are a mysterious non-human race that were reportedly the original inhabitants of the continent of Westeros. They were already living in Westeros when the First Men migrated to the continent, 12,000 years before Robert's Rebellion. According to legend they were last seen during the Andal Invasion 6,000 years before the War of the Five Kings.
They resemble children in appearance, hence the name:
The Children of the Forest were said to be humanoid, but when grown to manhood they were no taller than human children. Their facial features are very rounded and soft, like a very small child. They also have disproportionately large and expressive eyes (like human babies'), which are set wider in their face than would be normal for a human child of the same height.
Their culture was associated with nature and they worshipped Old Gods of the Forest. They also had access to strong magic.
Initially they fought with the First Men when the latter appeared in Westeros, but eventually they made peace with them and both races coexisted.
White Walkers are actually their enemies and they helped people of Westeros during the invasion:
The Pact lasted for two thousand years before the enigmatic White Walkers invaded from the uttermost north, bringing death and destruction to both races. The Children of the Forest allied with the First Men to drive back the White Walkers in the War for the Dawn eight thousand years ago. After the defeat of the Others, the Children, much-reduced in number, are said to have helped Bran the Builder raise the Wall with their powerful magics to prevent the White Walkers' return.
Those are children of Forest:
The Children of the Forest are a mysterious non-human race that were reportedly the original inhabitants of the continent of Westeros. They were already living in Westeros when the First Men migrated to the continent, 12,000 years before Robert's Rebellion.