People attending football matches have to queue to gain entry, climb the same stairs together, often touching the same railings, and use the same facilities (kiosks/toilet), where hygiene has never been optimal, and then you could be sat in front of someone coughing his head off and not have the option of moving elsewhere.
While children may not be generally affected in terms of mortality, they are presumably highly likely to catch the virus and then spread the symptoms, either elsewhere or in the home, which in turn raises the risk of older family members developing the virus and adding to the pressure on the NHS.