The health benefits of physical activity are too many and too important to let go. Physical activity should be built-in the daily schedules of children right from their infancy and there are many simple ways of achieving that.
Here is an American Academy of Pediatrics (AAP) chart to give you an idea about how to go about achieving the goal of keeping the children active, right from the time when they are just babies!
Here is another chart from Australian guidelines, which are more or less similar to AAP, but it also adds examples of common sedentary or inactive behaviour patterns to avoid at different ages.
Here are UK recommendations for physical activity for 5-18 Year-Old.