If u don't like reptiels like me you should try a hampster, though they require more up keep it's also a fun thing to clean the cage with a parent and take care of ther very own pet but take precautions because they can run all around your house and you must pay attention to ther water supply daily!
You should teach her that turtles are meant to be in the wild, not stuck in a tiny cage. A turtle in the wild can live to be a 150 yrs old or more, one in a small cage only live to be on average 6 months old.
Be careful of the kind of turtle. Some states have certain turtles or tortoise on the endangered species list. Being caught with an endangered species could mean hefty fines and possible jail time. Try not to take any turtles from ponds, lakes, or the wild just incase. Also, I would advise you to do a little research of your own so you will have a little knowledge when looking at pet stores. Try to go to a well renown pet store. Don't buy one from an individual unless you know yourself what kind it is. You don't want someone selling you a endangered species so they can make a buck. Try this website www.petturtle.com and see if that helps.
I do not see anything wrong with you buying her the turtle that she wants. Turtles are good pets, they are harmless and it is easy to maintain it. You may buy her another pet and she rejects it. You can talk to her and find out another pet she would like apart from turtle. This is, if you do not like the idea of a pet in the water.