Lots of private studios and chains are adding more classes for kids, it seems.
The little segment on Dancing with the Stars showcasing youngsters certainly won't hurt the popularity either.
Though the instruction may not always be as top-rate, I wouldn't rule out a community recreation center or park district setting to start with -- just to see how your daughter takes to it, and is also is possible she would be around a few more kids.
In the meantime, get her excited by watching the DVD "Mad Hot Ballroom" together. It is a great documentary about ballroom dancing classes in New York City schools. Listen to what the kids say about it. Very uplifting.