Dear Baby North,
I would not put a child under general anethesia unless if absolutely neccessary. While T2* is a very useful test, it may be worthwhile trying it without anesthesia. I usually sit next to my son when he has MRI's done. They provide him with a headset to block out noise and talk to him throughout. We have been doing this test for many years. I would suggest telling her what to expect, even make it an adventure for her - she may do well with the MRI.
I am happy to hear that your daughter's ferritin is in such a health range, likely her T2* will be good as well - but it is always nice to make sure iron is not depositing in the heart -(very unlikely given her age and ferritin levels).
Sharmin