Icy Thomas-Day was feeding a treat to the dog - a neighbour's pet - when the incident occurred.
The dog, Alfie, mauled and clawed at her face, leaving her covered in blood and screaming in terror.
Her mum and dad, Jessica and Paul, rushed her to hospital where she had surgery for her horrific injuries.
The family had been attending the dog's' third birthday party at their neighbour's house near Weymouth, Dorset when it attacked.
Icy's parents are demanding the owners have the pet destroyed, but the police have advised them they cannot enforce this as the attack happened on private property.
Mrs Thomas-Day, 31, told reporters: 'We had been there a couple of hours and Icy knelt down to feed Alfie a dog chew. All of a sudden there was this hideous high-pitched screaming noise and I looked down to see Icy lying on the floor covered in blood and screaming horrifically.
'The dog had jumped over the treat and lunged at her and tore and bit into her face, scratching at her six or seven times. A chunk of her left eyelid had been ripped off. It won't grow back again and she will have no eyelashes on that eye.'
Mrs Thomas-Day said it was 'disgraceful' that no action will be take over the incident, saying:
'It's disgraceful that the authorities are powerless to act over this. Dogs have more rights than humans. Everybody thinks West Highland Terriers are cute, little fluffy things. But this one went for Icy without warning or provocation.
'It wasn't in a corner or being teased, Icy as just trying to feed it. The dog's owners were mortified when it happened. We don't blame them for it, but they are adamant they are going to keep the dog and have asked us two or three times if we are going to pursue the case for having it put down.'