David Ball, of Middlesex University, provided evidence for the defence of James Porter, the 66-year-old headteacher and owner of Hillgrove School in Bangor, north Wales.
Kian Williams, a three-year-old pupil at the private nursery school, had leapt off the fourth step of a playground staircase, pretending to be Batman. He suffered head injuries and died a month later in August 2004 after catching a hospital infection.
Professor Ball said Health and Safety Executive inspectors had misinterpreted supervision guidelines. "Were HSE to have its way, I suspect that schoolyards would all look like factories with railings, crush barriers, self-closing gates, age segregation," he said.
Mr Porter, who has run the private school for 30 years with what the judge called an "enviable safety record", is appealing against his conviction. He was last week ordered to pay pound;20,000: a pound;12,500 fine and pound;7,500 in legal costs after a jury found him guilty of failing to minimise health and safety risks at Hillgrove. Local media also reported that he had agreed to pay a five-figure out-of-court settlement to Kian's family.
Kian's grandfather, Ken Williams, told The Daily Post in Wales that no fine was big enough to make amends for the loss of Kian's life.
Professor Ball said the Health and Safety Executive had wrongly prosecuted Mr Porter on the grounds that there had been only one teacher supervising 59 children in the playground.
The executive cited 1989 guidance which recommended one teacher for every 13 children in a private nursery school. But, according to Professor Ball, that ratio is to satisfy overall education goals and not intended to be strictly enforced at every moment of the day.
A study last year had shown that teachers at primary and secondary schools across England and Wales were each supervising between 33 and 99 children at break. "A little boy jumped down a couple of steps," said professor Ball. "However many supervisors you have in the playground, you can't stop something like that.
"It is getting to the point where we are putting our kids in sterile, flat, fenced playgrounds that, if they were in a zoo, would have the animal rights groups up in arms."
But an HSE spokesman said Hillgrove School could have done more to minimise the risk: other private nurseries in Wales had acceptable supervision ratios, and since the accident Hillgrove had installed a gate at the top of the staircase.
Steve Scott, HSE principal inspector, said play helped children to learn essential life skills, including managing risks.
"This case is not about restricting play for children at school," he said. "But a balance does need to be struck between the risks and benefits, and very small children should not be left effectively unsupervised."
John Dunford, general secretary of the Association of School and College Leaders, said heads were now expected to work in a "no-risk culture". "Standards expected of schools are now higher than those that most parents would observe at home," he said.