Film-maker David Bond believes we have a problem. We are raising a generation of children who don't play outside any more. Time spent playing outdoors is down 50% in just one generation.

Inactivity and obesity mean children born today have a lower life expectancy than their parents, for the first time ever. Meanwhile, evidence overwhelmingly suggests that natural environments have restorative physical and mental health effects, as well as specifically promoting child development. So children not playing outside, and not engaging with nature, is a big deal.

But David Bond has a solution. And it began with a film. Project Wild Thing premièred at the Sheffield Documentary Film Festival in June and starts a nationwide run in PictureHouse cinemas in the autumn. In it Bond is compelled to act when realising his young daughter (aged five) and son (four) spend the vast majority of their time indoors in front of screens. He appoints himself the "marketing director of nature" to take on the big brands that vie for children's attention and beat them at their own game. So begins a tour of the UK talking to academics, branding experts, parents and children to understand how we can reconnect with nature.

You may recognise Bond. He's done a similar thing before. His 2009 film Erasing David saw him attempt to escape the clutches of the surveillance society. But casting his endless energy towards his latest preoccupation, parenthood, may have spawned something more than just a film. Out of Project Wild Thing has emerged The Wild Network, a coalition of organisations eager to encourage children and parents into the great outdoors. And given that it includes the RSPB, National Trust, Woodland Trust, Play England, Play Scotland, Play Wales, the NHS Sustainable Development Unit and ad agency AMV, it just might have a chance.

"There are so many barriers that we've erected between children and nature", says Bond. "The Unicef finding is that children are happiest when they are with adults they love in a natural setting. Yet children between 8 and 12 spend on average four hours a day on screens... Children's roaming area has decreased by 90%... If we could slow this rapid decline in time spent outside and at least bring it to a stop, that would be an amazing achievement."

Big corporations are also in his sights: "I was using an Apple laptop in the film – that's a classic natural logo being co-opted to market a non-natural product. We need to ask these guys to chip in and operate with us. Anyone who uses natural logo to market a non-natural product should have to pay a tithe – 0.01% of their marketing budget, that the Wild Network can use to advertise nature."

The Wild Network has launched a free Wild Time App, giving ideas for outdoor activities based on the amount of time someone may have free, from 10 minutes up to half a day. Maternity packs stressing the importance of time outdoors for infants and new parents are being rolled out to maternity hospitals across the UK.

"I was shocked that when you have a baby you get given the bounty pack, saying you should use this brand of nappy or this clothes detergent – but nothing about the outdoors at all", says Bond. And a Wild Thing pledge asks individuals to spend as much time outdoors as we do in front of screens. Bond has taken it himself, though admits it's only feasible on weekends: "for two days out of seven you balance an activity that albeit fun has no active health benefits, with something that is overwhelmingly shown to stimulate children's and adults' brains."

There is a slight love-hate relationship with technology going on here. On the one hand screens are the enemy, on the other The Wild Network's very first offering is an app. Bond argues it's not a contradiction: "We spoke to a number of play workers who talked about how children raised on technology make iPad style gestures on live animals, going up to animals and making the finger pinching gestures you would make on a screen. And that's worrying... [But] we're really eager not to be anti-technology because so many amazing apps are being developed now which are reconnecting children back to nature... Stargazer for example really engages people with the outside world."

He also refutes any criticism that this is easier to address for middle class parents in leafy areas; more so again for those in the countryside, than in the city. "We found kids in well-off families who are incarcerated in-front of x-boxes because their parents are too busy with their high profile jobs to spend time with them", says Bond, "and children who are growing up in relatively impoverished situations who are deeply embedded in nature... I don't want to solve this problem with my family by taking them to live on the Isle of Eigg, as wonderful as that would be, because I think we've got to make this work in cities. Lots of countries seem able to do that, to make streets that are safe for children and cars, parks that are accessible, and wild life corridors."

Bond believes that even the smallest urban park can provide a valuable wild experience. A scene in the film shows inner city children in Tottenham Hale climbing trees for the first time in a local public space; before the Wild Time App it had simply never been suggested to them before. "The ultimate test is, look up, is there a ceiling?" says Bond. "Being outside is the first great step towards connecting with nature. I really see the front door as the threshold."

The Wild Network's backers appear equally enthused. Speaking at the film première, Andy Simpson, head of youth and education at RSPB, and now chairman of the Wild Network, said, "we've never had a resource like this that so clearly articulates the problem, and we and all the other partners in the Wild Network view this as the start of... a massive campaign targeting the next general election, targeting parents, and creating opportunities to make it easy for people to get their children out into nature. We think this is a ground-breaking moment in history for connecting children with nature."

The question is, will parents agree? Or are we now a nation that feels safer indoors?

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