I say if you want to take her, take her, no matter what the reasoning. That is something that only you and your family can decide on. I wouldn't worry about the age difference--when we were there last my 3.5 year old went on Tower of Terror and had no problems with the shows, so I don't see the age gap as a problem. Besides, there is a 15 year age difference between my oldest sister & my little brother--8 years between my & my brother with no one in between.
I would make sure the reasoning for taking her is thought out & made clear to her parents and then her. Ask the parents first, without her around. If it is a go, decide on what you will pay for & what her parents need to pay for (decide this before asking her parents). Costs include admission to everything, food (decide how, if, you want to split), souveniers, airfare--you should pay for hotel costs & car travel expenses, imo. Make sure you have her doctors info, a notorized form giving you consent to act as guardian and give medical care (be sure to put the effective dates in this letter), have a list of all allergies & know other medical preferences. Make sure you give detailed itenary & lodging info to parents.