Given that she is so light I would say the 8m would be a good place to start. Obviously in that light winds she might be a bit under powered but that kite will last her well once she is up and riding. If you have access to a 10m it would be best to start with a bit more power in the kite as that allows you to take 4-6” of trim out of the trim line to ensure that even if she is super tense and pulls the bar all the way into her she will keep the kite flying forward and have enough juice to be pulled and get going…
I recognize that likely leaves you a bit confused. My overall opinion is that in terms of a purchase the 8m is her best bet as it will be plenty for her once she can ride. Having access to a 10m to learn on would be nice though.