It was just a shot that Abrams wanted because it looked cool. Some people are able to roll with it, some aren't. I'm in the latter. Even in fantasy, some rules of reality still apply. And it's not like it's difficult for kids to understand. They know that stuff far away is harder to see. It can still be realistic and still be understandable. Besides, Alderaan wasn't a huge spectacle to the rest of the galaxy when it got blown up.
What they should've done is have Rey react to it. She could be like "I think something bad has happened," or something and then everyone starts to get reports about Hosnian being blown up.