A B&Q branch is investigating allegations that a young disabled child suffered a head injury after falling out of a cart at the store.
Tiffany Zimbabandu's mother says she was in shock after suffering a concussion following the accident at the Cheltenham store.
Four-year-olds with Down syndrome and skin condition vitiligo were rushed to the nearest A&E after the fall.
Her mother, 40-year-old Michelle Owino, told Gloucestershire Live that the tot "cracked her head" and felt her back badly.
Mother and daughter were sent home from the hospital with a handful of prescribed painkillers and a pamphlet on head injuries.
Doctors at Cheltenham General Hospital told the concerned mother to keep an eye on Tiffany for at least 72 hours.
Sir. Owino claimed the incident will leave Tiffany so scared to drive in carts that she will have to push her around the shops in a wheelchair, making it "very difficult" to do her shopping.
She: "I'm not happy. I'm writing a letter because she has Down syndrome – it's very difficult.
"It could have affected her throughout her life in that sense of shock. She doesn't feel safe going in the car anymore, so this makes things very difficult for me.
"It is absolutely disgusting what has happened."
Also suffering from common hypermobility syndrome – also known as double joint disease – Tiffany occasionally relies on a wheelchair to get around.
The medical condition affects her balance and often causes her further pain.
Her mother was in B&Q, in Golden Valley Retail Park on the outskirts of Cheltenham, shopping for pearls for her laminate flooring at the time of the accident.
Owino, a full-time caregiver for her daughter, stuffed her into a child seat in a carriage.
While his back was turned, the seat cracked of plastic, sending Tiffany crashing into the metal truck's bed.
The distressed girl "screamed" as Mrs. Owino rushed to help.
After the accident, her mother said she was "lucky to be alive".
She said: "She had fallen back. Her head bounced down to the bottom of the carriage.
“The back of the seat on the wagon stuck to the back. The plastic piece where she sat on her back was completely snapped away from the metal.
"She banged her head hard. She was in terrible shock. You don't expect a seat to give way to a child. It could have killed her."
No staff came to help Ms. Owino and her oldest daughter, 16-year-old Chantelle, she claimed.
B&Q has undertaken to investigate the cause of the accident.
A spokesman for the company said: "We can confirm that there was a customer complaint involving a cart location in our Cheltenham store on August 13, 2019. We are investigating the use of the cart to respond to the complaint."
Ms. Owino is in the process of filing a formal complaint with the DIY giant.