A woman, who Rebecca estimated was in her 60s, was pushing the trolley as it hit her son's head.
"I either grabbed the trolley or grabbed their hand, I can't remember, and said 'Oi'," Rebecca said.
The woman then started abusing her over her son's behaviour.
"She said 'Take him bloody home! Be quiet! I don't come here to listen to this!'."
Rebecca's mother encouraged her to lay a formal complaint with the store, and with police. She went back to Kmart and went through the footage with the store manager.
"It was sped up, but he pretty much said to me 'yeah you can see she'd kind of pulled the trolley to the side to direct it straight into him'."
"He slowed it down to show us, then said 'yeah we have enough, we'll make some complaints and see if we can trace her'."
When she got home, she received another call from the Kmart manager.
"He said to me, 'I'm sorry to do this to you… but I feel it's the right thing to do'.
"'We've reviewed the video footage in detail and we can see that she has actually punched your son in the head with quite some force, when your back was turned'."
Rebecca said this was "unbelievable".
"The whole lot was."
A police spokesman today said after speaking to both parties and making a number of inquiries there was "insufficient evidence for the matter to proceed".
The evidence gathered was not regarded as sufficient to provide a reasonable prospect of conviction when presented in court, he said.