Youngsters are not getting enough opportunities to play outdoors.
A study carried out by the Seattle Children's Research Institute and University of Washington found that around 50 per cent of kids do not have parent-supervised outdoor playtime every day.
Children who take part in physical activity are more likely to be healthy, as it can improve motor development, vision, cognition and vitamin D levels.
Fiona Reynolds, director-general of the National Trust, echoed this sentiment when she stated youngsters can have fun and "learn about the world around them".
The research – which has been published in the Archives of Pediatrics & Adolescent Medicine – found 58 per cent of kids not in child care go outdoors every day.
"Our results highlight the considerable room for improvement in parent-supervised outdoor play opportunities for preschool-aged children", the authors noted.
They continued by saying a special effort needs to be made to encourage more girls to get some fresh air, as they are currently less likely to indulge in physical activity than boys.