Some day -- like right now -- the kids are not going to know how drastically different things were even during the recent past. (Omigod, you mean people used to use their phones for, like, talking?!) The best way to give them illustrations is to show how some thing that's existed all along has changed over time. If something exists and then doesn't, or once did not exist and then does, I don't think they know what to make of it as far as understanding the flow of history.
The Super Bowl has been around long enough and is so popular that they'll know what you're talking about. And they've sung the national anthem just about every time. Since the major cultural shift starting in the early-mid 1990s, the performance has grown so Victorian in its overly ornamental encrustations that fairly recent performances, say during the '80s, look like they're from another planet. They're not plodding and spare, like Harry Connick Jr.'s 1992 performance. They have just the right level of ornament and oomph.
Moving away from that optimum level of ornament could go in either direction -- toward the puritanical minimalism whose most famous example is the Apple look, but also toward the super-emo warbling of much pop music of the past 20 years. There's nothing contradictory about both extremes being popular: they have in common a strong departure from the optimal middle ground.
In case you're fortunate enough not to have heard the more recent, overly embellished and drawn-out national anthems, here are a few out of many examples from the Super Bowl: the one that ignited the trend, Whitney Houston 1991 (and even this early one doesn't have so much ornamentation), Mariah Carey in 2002, and by far the worst Christina Aguilera in 2011. About the only halfway decent one of this period was the Dixie Chicks in 2003.
The latest one I could find before '91 was Neil Diamond in 1987 -- talk about brief and to the point! (Note for the kids: not Neil Young, the loathsome hippie/grunger.) Then there's Barry Manilow in 1984, and Cheryl Ladd in 1980. Not as bare as it would be if sung by Norah Jones, John Mayer, or any indie band, and still nowhere near as rococo as recent ones.
Perhaps the most dramatic example of how different things were is Diana Ross in 1982. She was a superstar from the heyday of '60s girl groups and disco, so based on her counterparts of the past 20 years, you might expect her to unleash her inner diva upon the audience. Yet she doesn't warble at all, and invites the crowd to sing along with her -- and you can really hear them! I didn't think about it until I saw this performance, but the more unique and unpredictable your rendition of a well known song is, the more impossible it is for the audience to join you. All you expect them to do is remain silent while they marvel at your awesomeness. If it's a guitar solo set within a larger song, that's cool. But not the whole fucking song, like it's mid-century jazz or something.
You get the picture, so I won't go through the whole history before the '80s as well. One glimpse from the '70s, though: here's the U.S. Air Force Academy Chorale in 1972. Again notice how "short" it sounds compared to the draaaaawn-ooooout ones, and how the low level of on-the-fly embellishment makes it easier for them all to harmonize.
Speaking of harmonies, here's the group that should have been in Barry Manilow's place, Huey Lewis and the News at the 1984 MLB All-Star game: