I'm with the posters who say that visiting and exploring will handle her perceptions of LA and the US in general. I say this as someone who grew up there: LA is expensive and over-crowded. LA also has excellent proximity to many different natural environments (beaches, mountains, deserts, everything!) and great weather. Getting around is a hassle, but your life can actually be really glamorous. And lots of people there are really attractive. Anyway, all of this is obvious to anyone who visits.
I went to NYC when I was 12. It was great! Just like the movies. Bustling, fast-talking, huge buildings, glamorous hotels and fancy stores. I also learned that while I love visiting NYC, I could never live there.
I now live in a smaller city in the US Midwest because I like never driving in traffic, a low cost of living where I could buy a house near the city with an acre of land, and friendly people. I have friends who rent tiny places in LA, work for pennies in entertainment, and LOVE it. One of my friends actually enjoys his traffic-y commute. As you said, different strokes!
I say have fun and let your niece dream! Youth is the time to let your mind wander with unrealistic expectations and vacation is the time to explore and learn. Enjoy some showbusiness if you can. Go see an improv show at the Upright Citizens Brigade Theater. All shows are under $10 and famous people randomly participate. Go see a free taping of a late night show. Unlike a sitcom, they don't do repeated "takes" so the taping is shorter, and they are taped during the day. The tickets are free.
As for the gun control threadjack, the right to bear arms is in the US Constitution. So the American relationship to guns and gun control is going to be very different than that of other countries. Most the gun deaths in the US are suicides (I think its 2-to-1 but not positive) and a significant portion of the homicides involve illegally purchased guns / unregistered guns. So the reaction to gun control is "why are you interfering with my ability to hunt / constitutional rights when it won't change anything?" I vaguely remembering researching gun violence in university for a paper, and found that even long before modern gun control laws (i.e. 1800s), the murder rate in New York City was overwhelmingly higher than the rate in London. Regardless, the violent crime rate in the US has been free falling over the last few decades. Why do Americans love shoot themselves or others while other places with high gun ownership don't have that problem (see Iceland)? Not an issue relevant to your vacation!