My 2 cents worth... My 7 year old DGD loves sewing on my 301. I have another vintage marked for her when she turns 10 but I am considering giving her the 301 on that date.
My 9 year old DGS enjoys sewing on the 401 that will be his next year. He has been sewing on it for two years. Since I repaired the boat canvas for our boat 2 years ago I know it is a workhorse and will last for him long into his future.
I also agree that the 66 and 99 are nice for children.
It will be so fun for you to see what she will make with her birthday present.