It's shorthand. It's a quick way to say "Super hero movies don't develop plots and characters that are very intricate."
"For Kids" is more of a way for someone to point out that "even a child can get a grasp of all the concepts being portrayed in this subject matter."
Typically, these people are 100% correct.
Superhero films especially, are meant to fit to a low thought, high energy, low trauma, high morality play.
There isn't a superhero movie made, ever, that has the same emotional impact as Shindlers List. There isn't a superhero movie ever made that has the complexity of plot as Memento. No super hero movie will ever let the hero be as immoral as Clockwork Orange's protagonist (hell, even most badguys are little more than terrorists with no real ideals)
If you pray to the altar that is superhero films and claim they are the greatest films of all time, you need to read more, and watch less.
If you just love a fun story and some great action, then fine. Have a favourite fun film - but don't try to convince others its "great film art".
