Children in care were punched, locked out naked and had vinegar poured on cuts, according to reports that were filed over three years before the homes were finally shut.
A BBC investigation has learned more than 100 concerns were logged at the Doncaster children's homes, which retained a "good" Ofsted rating.
Leaked documents also show Ofsted was alerted 40 times about incidents.
The regulator has now apologised, as has Hesley Group, which ran the homes.
More than 100 of the UK's most vulnerable children in care are feared to have been harmed, involving many who are non-verbal.
The homes, which included two residential special schools, charged local authorities £250,000 a year to care for each young person.
The homes continued to be rated as "good" by Ofsted, but in March 2021 the regulator finally stepped in and they were closed shortly afterwards.
BBC News revealed in June 2022 that more than 100 safeguarding reports had been made to authorities and, in October, an expert panel said there had been "systemic and sustained abuse" in the homes.
But now - through leaked Hesley Group documents, including confidential safeguarding reports, and interviews with 15 former staff - the BBC can reveal the scale of the failure of authorities to act. Our investigation found:
Children were reportedly locked overnight in bathrooms, left in soiled clothes, made to sit in cold baths and deprived of medication for days
The mother of a girl - who is autistic and has severe learning difficulties and epilepsy - told us her daughter had been dragged across the floor of a home
Criminal record checks were not signed off for some staff for up to six months after they started working with vulnerable children
South Yorkshire Police - currently investigating some Hesley staff for alleged abuse - was warned by support workers three years before the closures
Hesley's accounts recorded a 16% profit of £12m for all the sites it runs - almost the same margin (17%) regarded as "excessive" by a government watchdog
Our latest findings have been described as the worst care scandal since Winterbourne View in 2011 by the providers' group, the Children's Homes Association.
The Hesley Group - owned by private equity firm Antin Infrastructure, which is better known for investing in gas pipelines - continues to run a school and placements for adults with learning disabilities. It says it cannot comment further because of the ongoing criminal investigation by South Yorkshire Police.
https://www.bbc.co.uk/news/uk-63792458
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Heads should roll for this, but they wont