I don’t know why people keep going on about the reserve, limited space etc. People with kids know that, anyone that’s hosted anything knows that.
It doesn’t seem at all that itself is the issue, and as OP hosts, she must know about limited numbers.
She’s upset, a bit as she said, at the public posting of the plan B list, and that the other mum verbally confirmed it.
No, limiting the number of guests isn’t rude.
The thing to do would be to not have the list public, nor say “oh yeah, someone else dropped out so you’re/your kid is filler.” Because that’s essentially what happened.
She should have kept that list to herself, just said that she’s glad to have her, and left it at that, when OP said thank you.
@Lizzylozzy444 just shrug it off and send your DD so she can have fun. She won’t know.