at the time, my youngest niece was 7, the only issue with her was that she was picking up on her dad's(one of my brothers) discomfort. It took a while for him to get over it, but after that, it wasn't an issue for her anymore and she would actually correct her dad when he used the wrong pronouns. Two other nieces that were I think 13 and 14 at the time were fine with it and wanted to start calling me aunt right off, they are my little sister's girls. An older niece, close to thirty at the time had no issues and both her kids, a boy and a girl, has only known me as I am now, so no worries there.
A lot of how kids handle it depend on their parents, if there are issues there, then the child may reflect the parent's discomfort when they pick up on it.