The schoolgirl was outside a house when the dog reportedly leapt onto a garden fence and jumped at the her.
Niamh's mother, Cheryl Webster, said: "I was absolutely terrified - Niamh's face and lip were just hanging off.
"She was on the pavement outside her friend's house when the dog came bounding out. It put its paws on the fence, then went straight for her face.
"There was a big loss of blood and they had to completely reconstruct her lip - she had to have over 100 stitches.
"The police spoke to the owners but, apparently, they don't want to put the dog down because they say it's never done anything like this before.
"But it's vital that the dog is put down - that's what I want more than anything."
Niamh, a pupil at Cotmanhay Junior School, near her home in Cotmanhay, Derbs, was waiting outside the house with a friend when she was attacked last Tuesday.
She was taken to Ilkeston Community Hospital by her parents and transferred to the Queen's Medical Centre, in Nottingham, before being moved to Nottingham City Hospital.
She was released three days later but Miss Webster, 39, said Niamh would not be ready to go back to school for a couple of weeks.
Miss Webster said: "We've had to take her back to hospital since she left because she was bleeding through her stitches.
"The injuries are, thankfully, healing well and little traits of her personality are starting to come back.
"But it was a scary ordeal for her and it worries me that others may have to go though what we're going through while the dog is still around."
Niamh's father, Darren Guy, 43, said: "No-one should have to go through the pain and suffering that Niamh did, or the heartache that me and Cheryl had to go through."
Bull mastiffs, a cross between a mastiff and a bulldog, were originally bred to help gamekeepers against poachers.
Under the 1991 Dangerous Dogs Act, various hurdles must be overcome before any prosecution can be brought.
A spokeswoman for the Derbyshire Police said: "We are still making inquiries at this stage to see if any offences have been committed."
(Telegraph UK - Feb 5, 2009)
