In my view, that was quite clearly intended for people who live alone to get out and about once a day or so for exercise, not for small kids who've been merrily mixing at school all day to go out yet again
You have no right to take views on what a law was designed to do, only what is written.
The government guidelines (which it's essential everyone follows remembers) say a minimum of 60 minutes every single day, and to avoid long sedentary periods, of the government recommendations only a few of the strength activities can be done indoors, and none of the suggested aerobic ones. The government guidelines say a school should provide 30 minutes of exercise in the school day (although the research suggests many schools fail at this) So that means your children need 30 minutes of exercise a day outside school.
So yes, children, if you want to follow government guidelines, should be able to do their exercise outside after school, perhaps with another person - there's good evidence that exercising with other people increases the value of it.
Again the NHS guidelines suggest:
"Get your mates involved. You're more likely to keep active if you have fun and other people to enjoy yourself with."
So remember, follow those NHS guidelines, as well as the law, and get physically active, and encourage your kids to be physically active, the law recognises that doing it with another is beneficial.