Kirsty McGuire, from Dover, Kent, said her son Codie McDowell was left screaming with pain.
The accident in East Sussex has led his family to warn of the dangers of barbecues on beaches to prevent it happening to anyone else.
But Rother council said disposal barbecues were banned from the beach.
Ms McGuire said: "He stood on it, yelped 'just get it off me, get it off me'  and that's when my brother's picked him up and all the bottom of his foot's just blistered."
She said when they looked at the sand they first realised it was bright red and then they saw the barbecue underneath it.
"How someone could just be so selfish and not put their rubbish in the bin," she said. "It doesn't take long to put a barbecue out especially with the amount of sea that's there and dispose of it properly."
She added: "I felt helpless. He was in so much pain and screaming and there's nothing that I could have done to prevent it and when he's laying there in hospital and he's 'mummy take the pain away', obviously it breaks my heart."
She said her son now needed hospital visits every three or four days for his injuries.
Councillor Simon Elford, tourism portfolio holder at Rother District Council, said:  "We were very sorry to hear at the injuries received by this young boy and wish him a speedy recovery.
"To ensure the safety of everyone using this popular beach, disposable barbecues are banned and our officers carry out regular checks of the beach during busy periods."

Steve McQueen's drama will play at more than 1,000 locations in the US and Canada from Friday, up from 411 last weekend, according to Variety.
The memoir on which the film is based has seen a similar boost, jumping more than 300 places in Amazon.com's chart.
Solomon Northup's book is now in 16th place, having previously been ranked in 326th place before Sunday's ceremony.
The title charts even higher on this side of the Atlantic, currently occupying third place in the Amazon.co.uk bestsellers' countdown.
12 Years a Slave was named best picture at this year's Oscars and won additional honours for supporting actress Lupita Nyong'o and screenwriter John Ridley.
The film has made more than $50 million (Â£27 million) in North America, according to the Box Office Mojo site, and has just been released on DVD and Blu-ray.
McQueen's film tells of how a free-born black man, played by British actor Chiwetel Ejiofor, is kidnapped and forced into slavery before finally winning his freedom 12 years later.
Interest in Northup's 19th Century memoir has been revived following the success of the film, with many public high schools adding it to their curricula and reading lists.
According to Screen Daily, film distributor Lionsgate is also planning to increase the number of screens that Dallas Buyers Club is playing on in the US and Canada.
The film, about an HIV positive rodeo cowboy who uses illicit means to obtain unlicensed Aids medication, also received three Oscars on Sunday.
Stars Matthew McConaughey and Jared Leto were named best actor and best supporting actor respectively, while the film received an additional accolade for its make-up and hair styling.
Meanwhile, the Walt Disney studio has announced that Frozen - winner of this year's best animated feature Oscar - has now made more than $1 billion at the worldwide box office.
It says the film is now the biggest non-sequel animated film ever released and the second biggest animated film of all time globally, after Toy Story 3.
A four-year-old child has been badly burned after he stepped on a disposable barbecue that was buried at Camber Sands while it was still alight.
[NEXT_CONCEPT]
Cinemas in the US are scheduling extra screenings of 12 Years a Slave after it won the best picture Oscar.