Anonymum40 I am very sorry your dd is having a hard time and I am sorry you are affected by this.
Just to clarify.... are you saying that you are long term friends with two mums and their kids are long term friends with your dd and other children, and they are doing something tomorrow for Halloween and your dd has not been invited? Also that this has been discussed in front of you and your dd and you are planning on possibly going on holiday with them all?
If this is the situation here is what I would do in your shoes....
Talk to dd first thing tomorrow and see what she wants you to do, just give her the option, mention it to one or both mums (separately - perhaps by phone) or not?
Then do what she wants you to do.
I must admit I agree with BertrandRussell when they say I really don't understand why you didn't ask the party mother.
I would be asking unless my dd said she really did not want me to. By not asking you are kind of suggesting this is OK.
It is certainly OK to have some activities that all children are not invited to, and of course sometimes places are limited by car spaces, or a very expensive venue etc. This does not sound like the case here.
Maybe they do genuinely think she is not free to join them!
I would ideally want to talk about this now even if it is not resolved before the actual event, e.g. in case it happens again (e.g. say at Christmas). My concern would be in not challenging this unfortunate situation you may end up bringing it up later and feeling a bit petty (which you would not be). So better to ask now (in a nice way, as someone has said before maybe 'have I missed something, have the kids got some problem I am not aware of?" .... sort of way.
Aside from this I would also encourage dd to make new friends, try out new activities. As I know you are doing.
I would be open with your adult friends away from the children, it seems that the girls are not including dd in things and I am not comfortable with this. I woudl also say I am not comfortable booking a holiday for next year. If things improve you could look at it again. It does not mean your younger children can't play together although I would be a bit uncomfortable about these two families if I have understood the situation right.
BUT make sure your dd is OK with all this because either way challenging or not challenging could have repercussions for her.
I do understand a bit as I was very shy at school and very dyslexic at school. DD is also very dyslexic and I have worked hard to ensure she maintains friendships and makes new friends, she is 10. By this I mean we have invited numerous friends back for play dates, meals and even a few for sleep overs. I have spoken to the school when there was any whiff of bullying or being left out (with dd's agreement to speak to school) and I have tied to encourage dd in things like going to Brownies/Guides etc to make friends.
I have done this because my parents did not encourage me and I was very shy and felt this would help my dd.
You may be doing this already and, of course, your dd is older than mine, and it does get harder to be involved in their lives and friends (rightly so) as they get older.
Lastly in your shoes I would get some counselling to address your own issues about the past so that hey do not affect you in your interactions with your dd or her friends or affect your ability to do things like this for her. It is not about you, you have not been excluded, she has. If there is a reason for this, any reason, then she should know about it. But ask her first, her feelings come first in this. All the best. 