Full-time mum Laura Tucker, 32, had applied false nails and fastened the top on the glue pot and put it on a shelf before she went to take the bins out.
But 18-month-old Santi Haines managed to reach the bottle from his highchair and removed the lid.
He then got out of the highchair and onto the settee before pouring the contents over himself when his grandfather turned his back for a second.
Laura and her dad Peter rushed Santi to the sink to wash the glue off, but were horrified to find it had spilled on his arm and reacted with his pyjama top, causing a large burn.
Santi was taken to hospital with third-degree burns so severe he had to have a graft operation – with surgeons removing skin from his bottom and transferring it to his left arm.
Distraught Laura claims shocked nurses told her the ingredients in the nail glue were the same as the hospital’s surgical glue.
Santi could be scarred for life and medics have warned it might take two years for him to recover fully.
Laura, from Reading, said: “The worst I thought it would probably do is stick fingers together.
“Santi cried but didn’t scream – the deeper the burn, the less it hurts, – but he was very distressed in the hospital.
“I was upset at first but now I’m more angry. I’ve found out it has happened to other people, and Primark hasn’t done anything about it.
“Primark have told people they were going to investigate the product, but they obviously haven’t.”
Laura had bought the £1.50 Primark ‘PS Brush On Nail Glue’, which has a sticker on it warning of potential skin ‘bonding’, to apply her own false nails.
But while she was in the garden with the bins, her dad Peter popped into the kitchen for a matter of seconds and returned to find Santi on the sofa playing with the open bottle.
Laura noticed her toddler son was scratching his arm and removed his top to find burns just below his elbow.
The little amount Santi had spilled onto the sofa was also started to smoke.
Nurses at Royal Berkshire Hospital’s A&E department were astonished by Santi’s injuries and couldn’t wash out all of the glue, even after two attempts.
Santi was transferred to Stoke Mandeville Hospital’s specialist burns unit in Aylesbury, Bucks., where he underwent a skin graft operation.
Laura was unable to be with her son when he was being put under anaesthetic or when he came round later due to social distancing measures.
She said: “We were worried because babies put everything in their mouths!
“I have no problems with how we were treated in hospital, but it did make it lots harder.”
Fortunately, Santi was allowed to return home immediately after the procedure.
Laura informed Primark but is yet to hear back from the company.
She also posted about her experience on social media, and was appalled when she received numerous messages from people who had also been burned by the product.
Laura discovered that some of the people who had also complained to the brand had only received a £20 voucher in compensation.
A Primark spokesperson said: “We were very sorry to learn of this customer’s experience and we are in touch with her to investigate her complaint.
“We take the safety of our customers and the quality of our products very seriously.
“This nail glue complies fully with EU standards for safety, quality and product labelling.
“The glue is made of two ingredients which are both commonly used in similar products sold by retailers across the high street.
“The ingredients are listed on the product packaging alongside the required warnings about skin contact.”
Santi is due to return to Stoke Mandeville Hospital next week to have his bandages re-dressed.