I believe I am both giving and loving, but I would just state to the mother as firmly but gently at the same time (for her children) that I am not prepared for extra guests. I hate manipulation by adults and her not checking this out ahead of time is just that. And I would try to make sure that her invited child got a broad smile and a happy to see you "come on in", as it is not their issue.
I don't know when disappointment for children became a bad thing.
But OP you had no issue with the extra guests, so you have already had some excellent ideas on how to deal with the shortage.