Heroism is a natural attraction for boys. As long as you steer the boy away from anything that might harm faith or morals associated with some of these heroes, I don't think there's much harm in it. Spiderman for instance is frequently depicted with the "el diablo" hand gesture. Batman can be a very dark figure. And of course some of the associated movies have immodest content in them.
Professor Plinio, the founder of TFP, constantly wrote about the thirst of young boys for the heroic. Once the boys become old enough to realize that these superheroes aren't real, they still aspire to what they stand for, and this aspiration can easily be translated over into the desire for TRUE and REAL heroism, such as happened with St. Ignatius of Loyola, having been all into combat heroism, and then having read the lives of the saints, recognizing where true heroism lie.
I let my son have superman stuff without a second thought. He's old enough now to understand that superman doesn't exist, but he still loves to pretend that he has all his "powers". That's just a natural boy thing and it's not only harmless but can even be good for their development. Pretending to fight and be soldiers, these are all the things that modern society is trying to strip from boys to turn them into effeminate Kaitlyn Jenners.