I would let her be the guide about this. I started shaving my legs without my mom's knowledge at this age because it was noticible, and the leg hair was dark, but she put me in this position. Age does not matter so much as puberty, which can happen between 10 and 16, depending on the kid, and if she has dark hair, why force her into a situation where she could end up being embarrassed about the way that her legs look? That self conciousness could carry into other things for the rest of her life.
I would let her try both, but I personally think a razor is more comfortable with a shaving cream, and it is easier for a young girl to do safely. Most girls that age just have a little extra hair from the ankles to the knees anyway. Get a good quality razor with at least 2 blades, the more blades they have, the less likely she will be to cut herself. One thing that you should teach her is good hygenic practice with anything that she uses, like being sure to rinse the blade out with hot water, let it fully air dry and inspect it for knicks and rust before she uses it again.