In Game of Thrones (2011), the Children of the Forest were the first inhabitants of Westeros, even before The First Men. They are said to not be human, though they do take a humanoid shape. They got their name from the fact that at full size, they only grow to the size of an average human child. During their wars with the First Men, many were killed. As time went by, many more died. Their numbers were small to begin with, and by current time, many people believe they no longer exist - if they ever existed at all. Almost no living person has ever seen them. They do, however, reveal themselves to Bran (Isaac Hempstead-Wright) and lead him to the Three-Eyed Raven who Bran has been hunting since the fall of Winterfell.
The raven had been appearing in Bran’s subconscious ever since his coma in Season 1. By the time the third season rolls around, Winterfell is destroyed and Bran is out in the wilderness with Hodor (Kristian Nairn) and Osha (Natalia Tena) protecting him. Bran meets a strange boy named Jojen (Thomas Brodie-Sangster) and his sister Meera (Ellie Kendrick), who encourage Bran to follow his visions, saying they are prophetic. Bran’s goal becomes a journey to discover the meaning of his visions and understand what is happening.
Who exactly the Three-Eyed Raven is has yet to be fully explained on the program. Until the very end of Season 4, he had appeared to Bran in dreams under the guise of an actual three-eyed raven, guiding Bran to a cave. Once Bran reached the cave, he realized the raven is actually a man whose body is fused to a tree. They have a rather ambiguous conversation that doesn’t answer exactly why he has called Bran to his company, or what his intentions are.
The official Game of Thrones YouTube page released a quick video on the making of the Children of the Forest from the Season 4 finale “The Children“. The final episode of Season 4 introduced the "The Child of the Forest," which ignited great controversy between the Sullied and the Unsullied because the Child of the Forest released fire balls in order to resue Bran, Meera and Hodor.