The most important thing to keep in mind is that she has to have confidence in her bat. As she gets older, and her swing gets better, she has to believe the bat she is using is the best. Also remember, it's the Indian, not the arrow. A better bat will help a good hitter with a good swing. A better bat will not necessarily make a not-so-good hitter good. Sometimes more credit is given to the brand of bat the hitter is swinging than the hitter's swing. Especially at the younger ages, a girl may have better success with a bat she feels is "cool" rather than a better performing bat. There is an old rule of thumb that I agree with...swing the heaviest bat you are able to "sucessfully" swing. With this in mind, do not go too heavy. At 9 years old I would suggest a lighter bat to not hinder the development of bat speed. Basic physics, the faster the bat is moving at contact, the farther the ball will travel. A bat with more mass (heavier) at the same speed at contact will hit the ball farther. A one ounce difference in weight may not seem like much, but that difference can make a huge difference in bat speed. My suggestion would be a 30" -10 (20 oz.) or a 31" -11 (20 oz.). It's been a while since I've looked at smaller bats, but I believe there are at least a few making a drop 11 weight. If she is able to "sucessfully" swing a 31" -10 (21 oz.) as suggested by Juliana above, you may get a few years out of that bat. Santa brought my DD who's starting her first year in college a 34" -8 (26 oz.) Xeno for Christmas this year. Yep she's a strong 6' tall power hitter who has developed her swing over the years to be able to swing a heavier bat "sucessfully."
Most of the higher end bats (Xeno, Demarini, Easton, Anderson, Worth, Mizuno, etc.) are all pretty equal in performance. The ball speed off the bat is limited by the regulating bodies. Most of these higher end bats push right to the high limit with their technology. So bottom line, it comes down to personal preferece in terms of feel and in a lot of cases price. You really can't go wrong with any of the higher end bats. There are balanced bats and end loaded bats, there are bats now "designed" for slappers. Most bats will come with a factory one year warranty. Generally only honored if purchased from an authorized dealer. A bat will not last forever, expect to get a maximum two years out of a bat. When she gets older and is perhaps playing school ball and fall tourneys, you should expect to be buying a new bat almost every year. Expect the bat to "break" at some point. Within the first year the manufacturer will replace it once. After the first year, you will be replacing it. Over time, most bats will slowly lose their "pop."
You can buy an aluminum bat at Wal-Mart for $35 or you can go top of the line new model of most brands for $350+. Many, many choices available.
If you decide to go higher end you can save a ton of money by searching for the previous year's model or even two year's ago model. In many cases dealers will dicount the old model to "get rid" of all the inventory so they can sell the same bat with a new paint job for $350 again. New models are usually minimally discounted, if at all. Sometimes last year's model you can find as much as 50% off or more. Watch Ebay and some of the bat dealers sites. In a lot of cases, the bats do not change from year to year. They take the same bat, give it a neat new paint job and put a $350 price tag on it. The bat I mentioned above Santa paid less than $150 for. It's a 2017 model (exactly the same bat as the new 2018 model ($350) except for the color scheme).
Good luck in your search and enjoy the ride!
