First you will need to strip down the room & assess the drywall situation… you’ll need to fix up any holes before you paint (you have boys you know there will be holes in the wall!) this will give you a quick DIY fix. Once this is done it’s time to move onto the painting.
Here’s a guide to picking the right kind of paint. As for color well I would go with the primary colors, yellow, red or blue…this doesn’t mean you have to go with the insane bright versions (well maybe on the red) so a light blue or yellow wall would easily fit this theme. If you want something even more subtle then do a grey….you can even make a city scape with masking tape! Pretty much any action hero looks good on this color scheme so that way you kid can mix in a few other heroes & not mess up the vibe.