Tenza were extraterrestrial species that use species from other planets to raise their young.
Nobody has ever seen the true form of a Tenza because they use perception filters to blend in their surroundings. The perception filter used by Tenzas includes altering the memories of their surrogate family so that the latter would accept the infant Tenza. Tenzas seem to have extremely powerful psychic abilities. George, a Tenza, was able to communicate with The Doctor, who was millions of light years away, through a message from a psychic paper. Also, George was able to actualize and project his fears by turning the people who frightened him into peg dolls and then trapping them into the dollhouse.
Tenzas are not only able to rewrite the memories of their surrogate families, but they are also able to rewrite their own memories as well such that they truly believe that they are part of the family. Once they have infiltrated a family, the young Tenzas would do anything to prevent their surrogate family from abandoning them, which unlocks their latent psychic ability.
Using their psychic abilities to send signals for help that can travel up to millions of light years and trap objects and persons that scare them can be explained by pure instinct to preserve one’s own genes and to ensure one’s survival. Tenzas are very obedient to their parents. The nature of a Tenza ultimately depends on the type of nurture their surrogate parents provide, including what they want the Tenza to be.
It is implied that Tenzas have some sort of spaceship to transport the infants into their surrogate families in different planets.
Millions of Tenza hatch in space and drift to different planets for prospective parents. George, a Tenza masquerading as a human child, is one of them. George misinterpreted a conversation he heard from his parents and thought that his parents would abandon him. This event increased his anxiety - giving him bad nightmares - and that transformed anybody that frightened him into a peg doll trapped in a doll’s house in his cabinet. When the Doctor came to help him, he and George's father became trapped in the doll's house when the Doctor learned the truth about George, due to his mother been infertile and unable to have kids. With help from the Doctor, George's father was able to reassure his son that he wouldn't be taken away and that his parents truly loved him - allowing George to overcome his nightmares and change everyone back to normal. When the Doctor left the following day, he said he would come back to check on George when he became a teenager, as that would be "when the weird stuff happens".