Parents are concerned about lack of freedom for unsupervised play
New housing estates should be car-free to let children play safely outside their homes, a debate has heard.
Organisers said making local streets safer for children could also help keep them fitter.
Local authorities, architects and housing associations attended the discussion at the National Assembly.
Sustainable transport charity Sustrans said it would not mean banning cars entirely but creating safe environments for play close to children's homes.
The discussion was chaired by the Environment Minister Jane Davidson, and organised by Play Wales and Sustrans.
Research by the Insitute of Welsh Affairs (IWA) for the BBC Wales season What are we doing to our kids? has shown parents are concerned about the dangers posed by traffic.
They thought childhood had deteriorated because of a lack of freedom to play unsupervised.
Marianne Mannello, the assistant director of Play Wales said the price of our "fast car dependency" had been the loss of our children's freedom.
"It is no surprise that the research undertaken by IWA highlights that traffic impacts upon children's opportunity to play outside.
"As a society we have failed our children, we have allowed public open space to become defined by our individual needs to move from one place to another as fast as possible."
"The over dependence on the car has also resulted in a loss of belonging, of being part of a wider community.
"If children know the characters and characteristics of the community in which they live, if there is a sense of familiarity and acceptance, they are more likely to be happy to play out," she added.
Sustrans said "car-free" did not mean removing traffic and parking entirely, but instead creating a immediate environment without traffic and separate parking.
Children would benefit by being able to play and travel safely outside their homes, they claimed.
The discussion was delayed following snow-fall in February.
But Lee Waters, director of Sustrans, said the weather had demonstrated the vision the charity had for car-free streets.
"The heavy snow that postponed this debate first time around was a very real demonstration of how, when cars are taken off the roads, children take the opportunity to play freely outdoors, socialise with friends and neighbours, and connect with their surroundings," he said.
"The lack of opportunity for children to play safely in their own street is contributing to an obesity epidemic which puts Wales near the top of the world's fat league tables," he said.
A spokesperson for the Welsh Assembly Government said it supported the principle of car-free developments, although not all locations would be appropriate.
"A range of assembly government responsibilities have an influence on the environment which our children experience. These range from play areas in and around our schools, to the massive new sports stadiums that have been developed in recent years.
"A quality, local environment, close to home is important for the development of our children, enabling them to play, exercise and learn about their area, safely and securely.
"The Welsh Assembly Government wants to see locally accessible areas for formal and informal play areas in developing communities across Wales."
That would be made clear in a revised technical advice note for planners and developers, they said.
Jonathan Adams, who designed the Wales Millennium Centre said regulation was needed to give developers a level playing field.
"It's not not just about designing housing - it's about the whole urban environment."
"What we should be thinking about are sustainable communities where people can walk to work.
"You've got to allow people to feel safe allowing their children to run around in the street," he said.
Sustrans is already developing three "DIY streets" in Cardiff, Port Talbot and Bridgend, based on similar schemes in the Netherlands.
The projects work with residents to find low-cost ways of improving their streets.