We're all for kids having a say in how their rooms are decorated, but it doesn't hurt to compromise and find a middleground. If you have comic book fans at home, check out these retro Marvel murals. Somehow retro makes everything all right. (And if they're just too big for you, they have smaller versions you can frame.)
As you can see from the photos, these murals are large - 48" x 72" - and would be quite the focal point in any room. Below are links to the three shown above at Art.com. For others and an even wider selection of retro Marvel artwork prints, search by "retro marvel" in Art.com's search box.