I just looked through the available skin packs on the Minecraft Marketplace and couldn't find any official Marvel or DC Comic related skins. There was a generic Superhero skin pack, but hardly seemed like something you might be looking for, especially if you have to pay for it.
It looks like you may need to take the custom skin route, which is fairly simple.
- On your fire tablet, in the browser, download a skin from The Skindex - Minecraft Skins (For some reason, my browser reports this site as being a phishing site, but trust me, I, and many others, have been using this site for years. It's safe; ignore the warning and continue to the site if it tries to block it. If you don't feel comfortable doing that, just Google search for other Minecraft skin sites)
- Open Minecraft: Pocket Edition.
- Click the hanger icon beneath the player model on the right side of the screen.
- In the "Default" section of the skin page, click the grey skin that looks like a blank canvas.
- The panel on the right should show a blank player model and a button above it that says, "Choose New Skin," click it.
- Navigate the gallery screen and find the Downloads folder. Inside that there should be the skin(s) you downloaded, click it.
- Once you are returned to the skins page, click the big Confirm button under the character model on the right of the screen.
- You should be all set.
Now when you play, you should look like whichever Minecraft skin you downloaded and installed.
Hope this helped, if it is still not what you were looking for, let me know, I will be glad to help further if I can. You might have an easier time reaching me if you PM me on here, as I should get an email notification right away - as I said earlier, I'm not that active on here anymore. If this didn't solve your problem, send me a link to one of the videos that he might be watching and I can get a better idea of what we're trying to accomplish.