Because there are so very many kids who go missing, (adults too), the news would be full of nothing else. They are usually known to run away and return. It is also likely to make no difference at all to the outcome whether the child is all over the news. And there may be good reasons why they actually shouldn't or wouldn't want to be plastered on the front pages.
The cases which are newsworthy are the ones where it is very unusual or foul play is suspected and it's thought press coverage will help.
Finally - we can all choose what news to read and people mostly lose interest. There are wars and scandals and new laws and crimes and celebrity dramas and natural disasters and silly "human interest" stories - and people take 30 seconds to skim them all.