Paul Smith pleaded guilty at Birmingham Crown Court over the death of five-year-old Charlie Dunn.
Charlie was found underwater in a 1.4m deep lagoon by children at Bosworth Water Park in Leicestershire in July last year.
Smith, 36, had denied any wrongdoing over Charlie’s death but he changed his plea during his trial.
Prosecutor Mary Prior QC said Charlie had been “left alone in a busy park at five years old in circumstances where there was a clear and obvious risk that he might come to very serious harm leading to his death”.
She said Smith and Charlie’s mother, Lynsey Dunn, had shown “ingrained and entrenched indifference” and “a gross failure to supervise not for seconds, and not for a few minutes, but for protracted periods of time”.
The jury had been told that on the day Charlie died, Smith was shouting: “For f***s sake, we’re ready to go. I don’t know where he f****** is.”
Charlie was discovered by an 11-year-old, who reportedly felt his hairas he was searching for his goggles. The woman who attempted to resuscitate him said it was the most “heartbreaking” thing she had ever done.
Charlie’s mother admitted to multiple charges of neglect, in separate incidents to the death of her son.
The 28-year-old admitted to a charge connected to failing to supervise Charlie in an incident between 2014 and 2016, which resulted in the child being found in a pedal car next to a busy road.
She also pleaded guilty to another charge of neglect relating to another child, who could not be named, in the summer of 2015.
More from UK
Man in court over alleged plot to blow up Number 10 gates and kill Theresa May with a suicide bomb
The plots to kill prime ministers
Storm Caroline: Met Office warns of potential risk to life as UK set to be battered by 90mph winds
Storm Caroline: What you need to know about the ‘deadly’ weather front
Poppi Worthington: ‘Botched police probe makes prosecution unlikely’, inquest hears
PC James Dixon: Theresa May leads tributes to Road Wars police officer killed in crash
Smith also admitted witness intimidation in connection with another incident relating to Charlie.
The pair will be sentenced on December 20.
© 2017, Vale of Glamorgan Broadcasting CIC T/A: Bro Radio & Sky News.