It sounds like a good idea to get the hamster early. She will be settled and tame by the time your nieces birthday comes around, with a little work. There are some good taming tips here
http://www.hamstercentral.com/wiki/Taming_tips
As for her hiding she is probably active when you are not about and is just very shy ans scared when you are about. Hopefully once she settles it won't be the case and she will instead be happy to see you. A lot of pet shop hamsters haven't been handled much and it is harder for them to get used to humans, but they het there in the end.
Good luck.