I just had to respond to your post!
RSVP...
Call/email on whether or not you will be able to attend.
RSVP Regrets Only...
Call/email if you will NOT be able to attend.
This RSVP thing strikes a nerve with me. 12 years ago in the city I grew up in I was "Formally" invited to become a member of the Cotillion CLub. For those that are not aware of this, it is a "coming out" bash for 16 year old girls ranked high in society by what we called "frilly and fufu" cake baker mom's that did not have anything better to do than watch what the girls in their daughters grade were doing in and with their life. After joining, it was the best thing I ever did. We were "formally" invited to something every month whether it be brunches, mom's and me's, teas, or dances. But first, we had to take an etiquette class. From posture standing, sitting and walking; greeting people; invitation and letter writing; and RSVP's!!! From then on, I have always followed what I learned.
So, why don't people do it anymore? I ponder the answer to this question all the time and every time I send something out that requires and RSVP. I just want to tell people that the door swings both ways. You may NEVER RSVP to anything, but when the time comes when you need a count by RSVP's, you gripe about people not doing it when you are culprit yourself. That is how I feel when I am sitting in your situation. I do a lot of party/shower planning for people and help them get everything ready for it and sending out invitations is what I HATE the most.
Well, I simply tell myself that other's are as busy as I am and they just forget and lose track of time. "Nicely Put"
If you are will and have the time, then here is my suggestion. You would need to put this together this weekend to give out on Monday...give them till Wednesday to RSVP.
On a cute little note card, card stock type paper do the following:
Are we going to see sweet little ??? at my daughter ???'s Birthday Party on Saturday, November 19, 2006? Please RSVP by Wednesday, November 15.
You could give the option to call or email. Attach a lollipop to it with some curly ribbon. It is something that will catch their attention and not be wadded up and thrown away. And it is not very expensive.
On inviting more kids at this short of notice...
It depends on you. My thoughts are that it is not polite as people may see it as their kids were left out the first time or something like that. Now, if you know their parents and can make a phone call in the next couple of days to invite them, then I would go ahead. Set a limit of additional invites like to 5 or something you think is reasonable for your budget.
And finally, your have the right to be FRUSTRATED about this!! Good Luck and let us know how it turns out.
K.