DENVER — The air was so bitterly cold here on a Tuesday morning this month that public schools delayed classes for two hours.

With the temperature around 12 degrees at 8 a.m., Jacquelyn Farnsworth said her husband, an electrician who usually works outside, was assigned indoor duty.

But Farnsworth’s daughter, Josephine, 3, doesn’t get indoor duty. She attends an outdoor preschool that doesn’t have an indoors.

Instead, the hardy kids who go to The Nursery School bundle up in layers of winter gear to head into the woods even on frigid days like this one. Several days a week, Josephine and her friends spend hours breaking sticks to build forts, digging holes in the ground and learning to wield tools — including knives and bow saws — that few American adults allow small children to use.

Their parents have embraced it.

“They’re running around. They stay warm,” Farnsworth said. “It gives her time to have free play with other kids.”

Timber Tillemann-Dick, 4, learns to use a whittling knife at The Nursery School in Denver. Rachel Woolf / for NBC News

There are no laws in Colorado preventing The Nursery School from keeping children outdoors in subfreezing temperatures or showing 4-year-olds how to use a whittling knife to carve sticks into spears.

That’s because The Nursery School is small enough — two groups of four students led by an instructor — that Colorado law doesn’t require it to be licensed. The school’s owners dropped out of a state pilot program aimed at licensing outdoor preschools after chafing against some of the state’s requirements.

The school’s decision to shun licensing puts it at the center of a debate confronting the outdoor preschool movement as it strives to move beyond its roots as an unconventional option for families willing and able to pay hundreds of dollars a month for their children to play in the woods.

Outdoor or nature-based preschools have expanded dramatically in recent years, from just a handful of programs a decade ago to more than 400 across the country today. But while the programs have gained in popularity, driven by parents who believe they teach independence and fortitude while promoting a healthy lifestyle, their growth has been hamstrung by state regulations that are designed to keep children safe.

Now, as outdoor preschools look for a way to expand, the schools and their supporters are pushing states to consider licensing programs, even in places where children can encounter wild animals or poisonous plants.

Licensing can be difficult, because “our system really isn’t set up for that,” said Christy Merrick of the Natural Start Alliance, an organization that promotes outdoor learning. She’s heard of licensing agents asking traditional preschools to remove trees from their playgrounds “because the roots are a tripping hazard.”

Two states — Colorado and Washington — have taken the lead on trying to bring these schools into the mainstream. Regulators there are developing rules that could lead to outdoor preschools becoming fully licensed. That could allow programs to accept state child care subsidies, which would make them more affordable, and it can ensure that as schools expand, teachers know how to operate safely. So far, regulators in the two states say there haven’t been any injuries beyond skinned knees and bee stings.

Outdoor educators do not always agree with states about what’s safe. The Nursery School co-founder Brett Dabb, for example, says an important part of his program is teaching children to make smart choices around fires and sharp tools — which Colorado regulators say they would never allow in preschool.

But Dabb says he’s glad the conversation is beginning; he hopes schools like his become more common.

“It’s about what children learn about themselves from being outside,” Dabb said. “It’s helping them to think critically, to navigate situations and to figure out their emotions around changing situations. It’s helping them to be resilient.”

Following the rules

As the snow swirled on the recent Tuesday morning and children wearing four pairs of socks and multiple layers under their snowsuits headed into the 123-acre nature preserve where The Nursery School holds its classes, Denver’s other outdoor preschool, the Worldmind Nature Immersion School, started two hours late.

Worldmind, which is now the only school in Colorado’s outdoor preschool licensing pilot program, adopted the same delayed start as the Denver Public Schools.