I see that the Head is now going to report back to you on Monday, after she has spoken with the other child and reached a decision, Karen.
I don't think this is good enough [actually, I am appalled that a Head would continue to shilly-shally around, putting the onus (of whatever way the decision goes) on two 6 year olds] and if I were you I would preempt this, by following up your Friday conversation with a letter, hand delivered before school, and marked for 'the attention of Head, very urgent'. Something like:
Dear Head,
Further to our meeting on Friday, I am writing to confirm that, while I am content that the solution to this unfortunate situation is that both DD and OtherChild should each play Angel in just one performance, I must however insist that DD does the performance that will be attended by Parents etc.
As you are aware, my Dsis will be attending that performance, all the way from Australia, an opportunity highly unlikely to occur again! We have been looking forward to this for weeks, indeed since the day we heard DD had been given the part.
I am aware that there has been concern expressed by both Yourself and Teacher about OtherChild being upset, however quite frankly I do feel that, from the outset, there was a marked lack of concern given over to the upset caused to DD by this whole episode.
I look forward to hearing from you later today that DD will indeed be playing the Angel in the Parents Performance and, beyond that, I would like to have a meeting with you at a later date, perhaps for your convenience after Christmas, to discuss the the best way forward for DD in this particular class.
Yours, etc
KarenOfArc64
If you do go down this route Karen (and I think you should) I think it would be better typed/written on an actual letter to be given into school before any classes start tomorrow (keep a copy) but if for whatever reason, it is better for you to do it as an email, then still go into school before any classes start tomorrow and tell them there is an email that Head must see first thing.