By Pat Hurst and Ryan Hooper
Ava-Jayne Marie Corless was asleep in bed at a house in Blackburn, Lancashire, on Monday night when she was savaged by the animal.
Police and ambulance staff attempted to resuscitate her but she was pronounced dead at Royal Blackburn Hospital a short time later.
The girl's mother Chloe King (20) and her partner Lee Wright (26) are being questioned on suspicion of manslaughter.
Lancashire Constabulary said the dog, which was destroyed after the attack, had been identified by experts as a pit bull terrier-type dog which people are prohibited from owning under the Dangerous Dogs Act.
Locals claim to have made previous complaints about the large dog's behaviour to police.
Miss King and Mr Wright were said to be downstairs when the attack took place while Ava-Jayne was in a front bedroom.