How many kids would have to be invited for this to be 100% reciprocal for all of the birthday girl's friends? It's a sleep-over. Could *you* do that for all of the girls in your daughter's K?
My sons *always* want to invite everyone in their classes to *everything*. My husband and I can neither handle or afford that. For my oldest, that would mean a sleep-over for 16 boys, ages 6-7! YIKES!
My middle son already *verbally* invited 16 pre-schoolers to *his* birthday party this summer! And all I can think is - what am I gonna do????? I'll probably roll with it, keep it simple, and hope half of them will be on summer vacation. But still....that's a LOT of kids.
The other little girl is 6, right? She's excited. When my kids are excited they cannot keep their mouths shut, even if they should. I almost always know what I am getting for my birthday, Mother's Day, and Christmas a few days beforehand - they just can't help it yet.
If you like the other little girl, just tell your daughter that her friend probably couldn't invite everyone and that's okay - that it doesn't mean she can't have fun with her another time.
None of us want our kids to be left out - ever. But the thing is, this probably isn't about your daughter at all. If it's not a consistent pattern of leaving your daughter out, if the little girl is usually nice to your daughter, reassure her that her friend is still her friend and sometimes this is the way things go.
Now, if it is *ALWAYS* your daughter who's left out, it's a different matter and time to teach her when to "move along" and focus on people who make time for her.
Best of luck to you!
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ETA after reading responses, I wanted to share this:
I wasn't asked to be in "my best friend's" wedding party. I lived 2000 miles away, was dirt poor, and was a bit flaky at the time. She did invite me, but didn't think I could come - though I did. Turned out, she had a ton of bride's maids and yes, my feelings were hurt.
But you know what? It was *her* wedding - it was *not* about me, and so, I was not a jerk about it. She considered me a close friend, but not her "best", and it's not like she meant to hurt me. I didn't put her on the spot, nor did I think she was low class. Making some other person's special day about you (or your kid) - that's, to my eyes, not classy.
It's been 20+ years, and I'm still close with that friend. If I'd decided to make *her* event about me and my feelings, I'd have lost out on that. And to be clear, she is a good friend who is there for me and never expects squat in return. She simply didn't include me once when I really cared about it.
So...I think your response should be guided by whether or not this little girl is nice to your little girl. Either way - it's good for to learn that honoring our feelings does not mean being offended and trying to make others feel awkward because they didn't do what *we* wanted.