What is more, furious mum Laura Lavelle said the dog’s owner started shouting abuse at her after the attack.
Harley was left with a deep bite wound and bruising and had to be taken to hospital for treatment.
Laura, 29, said she thinks the dog was a pitbull terrier.
She said: “As I turned around I saw the dog lunge at Harley, get hold of him and pull him across the floor.”
Mum Laura was strolling along Bognor seafront about 3pm yesterday with her two daughters and Harley in a stroller.
She did not initially intend to go into Marine Gardens, but said her girls wanted to do some cartwheels so they went in.
Laura, who lives in Aldwick Road, said: “Harley said ‘mummy can I go?’.
“So I got him up so he started skipping along getting excited because he was closer to his sisters.”
Laura said she noticed a woman sitting on a bench nearby with two dogs that looked like pitbull terriers, but Harley was not near them.
What followed traumatised her and has left her afraid of what could happen to Harley in future.
“I saw the dog lunge at Harley, get hold of him and pull him across the floor.
“The dog grabbed hold of his ribcage.
“Harley’s head sort of went slo-mo. He got swung like a rag doll.”
Shocked, Laura ran over and rescued him from the dog.
“I looked down at Harley’s clothes and I could see blood.”
Harley was screaming as she got him away from the dog.
Laura was shocked when the woman with the dogs started shouting at her, she said.
Laura said: “She did not apologise. There was nothing, no emotion there.
“She started following me, shouting abuse at me.”
At this point her two daughters had seen their mum and Harley upset and were crying out too.
Laura said the woman eventually turned around and walked off without apologising or seeing if Harley was alright.
The police were called and Harley was taken to hospital by ambulance.
There was some confusion between Laura and other eye witnesses as to the dog’s breed.
Laura believes it was a pitbull but others thought it might have been a Staffordshire Terrier.
Police have released a picture of a woman they would like to speak to in connection with the incident.
Sergeant Matthew Phelps of the Arun and Chichester Prevention Team said: “The boy has been left very traumatised by this incident and we are providing support to the family.
“If you recognise this woman or was in the area at this time and saw a dog matching this description please get in contact with us immediately.”
To report information visit the Sussex Police website or call 101 quoting reference 840 of 06/08.