If you really want to bring in new readers, putting comics back into grocery stores isn't the way to do that. The fact of the matter is kids today don't really want physical media; they want things they can look at on their phones/tablets.
The future is in digital comics, the comic companies need to look into making the digital comics cheaper than the physical copies. I know that's irksome to most people, but it's the way the world is going.
As for Nighthawk, maybe Marvel could have promoted it a little more (did he show up on Ultimate Spider-Man, Avengers Assemble, Guardians of the Galaxy?). Maybe do some animated shorts on Disney (or Disney XD) website or even YouTube. Heck, do a :15 ad on YouTube. Maybe that would get kids attention.