K.S.
I wouldn't mention anything about siblings- people who otherwise wouldn't have thought of it will suddenly treat it as an opportunity to have someone else watch their kids (whether they stay there or not). Most people will ask if it's ok to bring a sibling.
I used to write something like "feel free to stay if you'd like, but drop offs are fine". Some moms aren't ready to leave their kiddos, so you're giving them permission to stay, but letting others know it's fine to drop off.
And either way on the rsvps. I really hate it when people don't do this. I think it is SO rude. If there's just a couple, I usually count them as a no because I know I would have enough food or whatever. Maybe have a couple of extra goodie bags ready in case. If it's a thing where you are going somewhere and paying per child, or there are a bunch that didn't rsvp, I would call or e-mail. Remember- THEY were rude, not you!