Rebecca said the child grew restless in the checkout queue before crying and throwing what she described as "a really bad tantrum - probably the worst I would have seen".
When Rebecca, her son and two older kids reached the top of the line, she said the 60-something-year-old woman pushed a trolley towards the two-year-old tot and yelled at her to "take him bloody home".
The mother-of-three said Kmart staff had shown her CCTV footage of the attack which she claims proves the woman intentionally steered the trolley towards her son.
After Rebecca took her children home, a Kmart worker phoned her with further distressing details of the assault.
The staff member said: "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."
Police are looking into the incident.