Everyone involved seems to be doing the right thing. To be honest, I think you should just all keep doing what you're doing and see how it goes. This other little girl almost certainly will grow out of this phase. And if she doesn't (ie. she has some sort of behavioural problem) the nursery and her parents are sure to pick up on this and get it dealt with. In the meantime, I think you just need assiurances that the staff are watching the other little girl if not all of the time then most of the time.
I speak as the mum of a DS who was 'the hitter' (and 'biter', actually ) at this age. It was mortifying, and the parents who dealt with it like you are doing - by being reasonable and mature - made the situation much easier to deal with for all concerned. The situation with my DS went on for several months and his nursery allocated a member of staff to 'shadow' him and try to stop the aggressive behaviour before it happened. i think it is reasonable that the nursery do this - and I don't blame you for wanting assurances that they are doing everything within their power to stop your DD and others from being frightened or hurt.