It sounds like she has done her research, just make sure you have done yours. It is not reasonable to expect her to be able to care for the dog 100% of the time. Once she gets in high school, she will be in extracurricular activities or will want to spend time with her friends and she should be allowed to.
My family always had dogs when I was growing up and I got MY first puppy when I was 7. The deal was, that as long as I was home at feeding/walking time I was responsible for it. That continued up until I went away to college and my mother took over as the care-taker. I personally think that if a kid is responsible, and the whole family is on board, that a dog is a great thing.
If, for some reason she ends up losing her commitment to this dog, I certainly hope that you will step up and take care of it instead of placing it in a shelter. Chihuahuas are flooding the shelters and it's odds of finding a new home will be slim. Make sure that YOU want the dog, too. Otherwise there will be problems down the road. If I had ever lost interest in my dog, my parents would have NEVER rehomed her because they loved her, too, and if they had given her away, it would have taught me that pets are expendable, and that is a horrible lesson to pass on.
Find a good breeder, (not on a website or on craigslist) or better yet, go to the shelter. I guarantee that you will find a Chihuahua puppy and it would be a great experience and lesson for your daughter to learn about rescuing dogs.