Here's my take on it, as a summer league swimming coach. I would get the kid into for a year, but rather than committing to both long course and short course, have her just do the short course (winter) rather than doubling up in the summer. After that, gauge her interest level. Odds are, when she goes back to summer league, she'll be a superstar, and will gradually get more and more into it as the years progress. Like has been mentioned, to keep from burning them out, look for coaches that really focus on technique rather than yardage. Sure, they might not win every race at the USS meets now, but you'll be able to see that they just look like a better swimmer. Then, when she gets older, she can start building up the yardage, and she will have a very good foundation on which to do so, and will turn out to be a very good swimmer.
But usually, as to the foundation, it's good to get them into it for a year. Then if they decide to take a year off to try something else, they won't lose as much as they would if they were yardage focused. Also, the technique focused teams usually aren't as anal about the kids being in practice every day no matter what, because they know that their training (at least at the youth ages) isn't entirely dependent on it.
You will be amazed at the difference in coaching that you see at USS from summer league. Yes, there are some very good summer league coaches (I've seen college coaches that coach summer league) but you just can't get as much out of the kids in summer league, because half of your time is spent getting the kids to quiet down and stop fighting. USS practices are more of a school atmosphere, mostly because of the fact that everyone is the good swimmer, and the coaches are allowed more leeway in how they handle distracting swimmers.
If you don't think you have enough knowledge to find a technique focused program on your own, talk to one of your trusted summer league coaches, and ask them to come sit in on a USS practice with you. I'm sure they'd be willing to do it for no more than the cost of a private lesson, and it'd definitely be worth it.
In summary, look for a technique focused program, have her do it for a season, and then play it by ear. Or come back and ask texags for more advice.