Parents must ensure that physical activity is part of the routine for children and families during the COVID-19 lockdown and all kinds of screens are kept out of the kids rooms during the sleep time.
The study, published in the journal 'The Lancet Child and Adolescent Health', covering 15 nations, found that time spent in places such as parks, beaches and community gardens reduced by nearly a third between the week ending February 23 - before the World Health Organisation (WHO) declared a COVID-19 pandemic - and the week ending April 5.
Travel by public transport was down by more than half - 59 per cent - over the same period.
"The measures against COVID-19 are in place for a very good reason but this reduction in physical activity could be seen as an unintended consequence," said study lead researcher John Reilly from the University of Strathclyde in Scotland.
"Even before the lockdown measures, it was a major problem; our previous research has found that, in Scotland alone, fewer than 20 per cent of children were meeting physical activity guidelines," Reilly added.
While these and other restrictions are in keeping with the global effort to halt the spread of COVID-19, the researchers found that they were having the effect of reducing still further what were often already low levels of physical activity in children.
The researchers make a number of recommendations to families, health professionals, teachers and policy-makers on promoting healthy activity, including taking the opportunity to go outdoors, while observing distancing regulations.
Incorporating physical activity into children's daily routines - supported by the use of electronic media - and breaking up extended sedentary periods every 30 to 60 minutes; families should also be encouraged to join in while observing distancing regulations keeping children's bedtime and rising time consistent.
Keeping screens out of the rooms where they sleep and avoiding screen use before bedtime, the researchers suggested.
"It's important that people make whatever use of their environment they can and take the opportunities they can to keep the physical activity going," Reilly said.
"We have found that they are much less active on the non-school days of weekends and holidays; our concern is that they are missing out not only in education but also in inactivity," the researchers noted.