Charlie Dunn - who drowned after being allowed to ''fend for himself'' at a water park - was placed on a child protection register a year before his death, a judge was told at crown court hearings.
Birmingham Crown Court was told how Charlie was dragged from a lagoon by other children minutes after his stepfather, Paul Smith, was seen swearing and blaming others for his disappearance in July 2016.
Smith was jailed for seven and a half years after admitting Charlie's manslaughter on the grounds of gross negligence, witness intimidation and driving while disqualified.
It emerged in court that Smith was a ''person of interest'' for the authorities after amassing 10 previous convictions covering 28 offences, including burglary and theft.
The 36-year-old was sentenced to five years and two months for the manslaughter of Charlie, who went missing while unsupervised on a trip to Bosworth Water Park.
Mrs Justice Jefford also sentenced Smith to a consecutive two-year term for threatening to petrol-bomb the home of a witness, and a further four months for driving while disqualified.
Charlie's mother, Lynsey Dunn, 28, was given an eight-month suspended jail term after admitting neglecting Charlie in a separate incident in 2015 when a neighbour prevented the then four-year-old from driving a toy car on to a main road.
Smith had changed his plea to guilty part-way through a trial which heard other children pulled Charlie from a lagoon after Smith was seen smoking and heard saying: ''For f***'s sake, we're ready to go. I don't know where he f****** is.''
Social services are known to have become involved with Charlie, whose death is being examined by a serious case review, when he was 14 months old.
The court heard Staffordshire County Council had noted ''poor home conditions, a lack of food and poor hygiene'' before Charlie was placed on a protection register in June 2015.
Prosecutor Mary Prior QC told the court Dunn was of ''limited intellect'' and had no previous convictions.
Mrs Justice Jefford said: ''I do not doubt that Lynsey Dunn and Paul Smith had genuine love and affection for Charlie.''
Rejecting Smith's assertions that he had been an ''impeccable'' stepfather, the judge told him: ''Nothing could be further from the truth.
''One father (in the lake at the park) had to explain to another that Charlie was not his son. You were completely indifferent to Charlie's whereabouts and safety.
''This was not a case in which there was an isolated and momentary lapse in care and supervision.''
Staffordshire councillor Mark Sutton, cabinet member for children and young people, said in a statement after criminal proceedings: ''This was an incredibly sad case and our sympathies are with Charlie's wider family.
''A serious case review is currently being conducted to examine what lessons may be learned by partners to help prevent such a tragic event happening again.''
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