Thank you soooo much for all your input and advice!! I would have been off calling the girl's mom apologizing and explaining the whole reasoning behind why we didn't invite her daughter - yikes!
Yes, just to clarify, there's 24 kids in my dd's class and we invited 11 of the 15 girls in the class, and 3 of the 9 boys in the class - making a total invite list of 14 of the 24 kids were invited. That means 10 of the 24 weren't invited. It would have cost us much more $$'s to invite 10 more kids and their parents because we have to buy tickets for everyone to come to the party at this place (which would mean 20 more tickets - about a $100 more!), so I would hope it could sound reasonable to the mom or anyone else invited that it would just be really expensive to invite everybody in the class.
I've posted on mothering before about the whole situation between the little girl who excluded my daughter from school, and it was their teacher who encouraged me to not make a big deal about the whole situation because it's "normal" behavior to be exclusionary with friends at 5. She thought it would be more troubling for my daughter to have the girl's mom and the teacher lecturing the little girl on how she has to be nice to everyone and include everyone in her play. The teacher said the little girl would just do what she'd want to do when no one was looking, anyways. It worked well for my daughter when we encouraged her to just make new friends and to find things to play and do on her own (which she never really did before - she lovvvved just playing with other kids, never really on her own before that).
The other little girl's mother was definitely aware of the situation through another mom who was good friends with the little girl's mom and witnessed the exclusionary behavior with me one day when we were both volunteering in the classroom. I don't know what the other little girl's mother ever did about the information she received about her daughter, but the mother never addressed me to tell me she was talking to her daughter about it.
Also, I learned over time that several other girls were being treated just like my daughter was by this little girl- so it helped me at least feel a little better that it wasn't just my daughter who was being singled-out by that little girl- she was just very exclusionary in her play.
As I mentioned briefly in my original post, my daughter and I did originally speak at length about the excluding situation (she cried and cried for weeks about this when it all first started happening - uggggh, it was painful!), and I encouraged her to privately pray for the little girl when she told my daughter to go away and to love her anyways because she didn't understand how she was hurting my daughter. I'm so glad that my daughter doesn't appear to be bitter or mad at the other little girl, but has rather just found a way to move on (we'd talked about forgiveness before, too), and I just felt like it was my responsibility as her mom to provide her with some boundaries on her birthday from having to have the other little girl at her party where the little girl could potentially exclude her again, or tell her to go away again, as usual, on my daughter's happy day. I just wanted to giver her a break from it all for the day, you know?
Thank you so much for reading and responding to my post. It's really kind and considerate of you all to take the time to do that, and I need to try to get better at finding pockets of time in my day to try and respond like that to others on here, too. You're so helpful, mommas!