I'm not saying junk food is good for kids. But kids are growing and their bodies change much more rapidly than adults'. If they slam a liter of soda a day they can still burn those calories off by riding their bikes or playing soccer for 2 hours after school (unlike an adult, they have both the time and energy to do so). They can't burn those calories off by streaming CS:GO on Twitch for 4 hours.

So you're falsely equating an adult's fitness regimen with a child's (kids don't care about showing their abs).