Taking a 7 year old to a comic store is like tasking her to a pg 13 movie.. . Not too surprised it didn't work out. Comics tend to deal with adult themes and are aimed at older people.
For kids that young seeing is believing. My nephew at 6 watched Star Wars over Christmas. You explain that it isn't real, they are actors in a play, playing characters - he's been in several plays himself - and then later on he says 'but it must be real, because how could they make a whole planet?'.
Presumably because he himself remembered how it used to be, and did not realise that the age bracket had shifted so dramatically.
Even US comics used to be kid friendly.
The shortened-to-the-point-that-it's-mostly-wrong version:
Prices went up for a variety of reasons, ordinary stores stopped stocking them, and the only way the companies found to move enough issues to avoid total disaster was to target men in roughly the 14-40 age range through dedicated comic book shops, embracing, in the process, a sub-genre of "grittier" comic story lines that had gained some traction.
Most haven't had TV with on-demand 24-hour access to child-targetted programming for nearly that long, however.
And even less time have they had portable, go-anywhere screens with the same thing.
And it's not about the price, kids would much rather play with a tablet than read a comic (or go outside, even).
I might just be an old grump, though.