The Tenza were a species of alien whose young were raised by other species.
Tenza had advanced, though subconscious, psychic powers, allowing them to live among other species. They used a psychic field and a perception filter to remove inconsistencies in their histories. One Tenza was capable of capturing people who frightened him, shrinking them to fit inside a doll house and eventually turning them into living dolls. He could also contact the Eleventh Doctor's psychic paper when he was millions of light years away. (TV: Night Terrors)
Tenza were born in space, where they drifted until they found a habitable world. They would find a family that they could live with, then infiltrate it. The Tenza infant would adapt itself to suit its environment and alter the memories of its surrogate parents and those around them. After that, the Tenza would live out its life as a member of the surrogate parents' species without conscious realisation of its true nature. Such Tenza had odd "tics" which drew attention; for example, Alex noted that George blinked too much when he was frightened, would stare off in to space at odd times and had never cried. If the Tenza felt threatened, its telepathic abilities played havoc with its environment to keep it safe. (TV: Night Terrors)
At some point after Claire and Alex learned that they were unable to have children, a Tenza came to Earth and adopted them as its surrogate parents. The parents named him George and raised him as their own son. Because of the perception filter, they considered him an odd child but had no idea he was really an alien.
Before he was eight, George's phobias started to develop. His parents tried to help him, but eventually decided he needed professional help. George thought this would mean he would be taken away from them. This set off his psychic defences. His phobias came to life in a doll house, which was governed by the peg dolls who terrified him, and abducted anyone who frightened him. He also contacted the Eleventh Doctor on his psychic paper. When the Doctor came to investigate, he realised that George was not human. This set off George's defences, pulling both the Doctor and Alex into the doll house. When they were attacked by the peg dolls, the Doctor tried to get George to help them. This pulled George into the doll house, where he too was attacked by the peg dolls. Alex saw his son in danger and tried to save him. This proved to George that his fear of his parents sending him away was unfounded and everything returned to normal. The Doctor left Alex afterwards, telling him that George would grow up to be whatever his parents wanted him to be, though Alex should watch out for when George hit puberty. (TV: Night Terrors)