My son is in reception and his birthday is coming up. Booked a soft play party with the mum of another child in the class whose birthday is the same day.
Invited the whole class - sent invitations a couple of weeks ago to give plenty of notice.
Our of 28 children - 11 have replied 
I don't drop off but the other mum does and both our numbers are on the invites.
So now, because a need to confirm numbers soon she is going to have to try and chase up in the playground and/or we will have to send reminds notes as we don't have all phone numbers or a Facebook group etc.
AIBU to think it's really not difficult to RSVP - I know people are busy but so am I and I still reply to invitations as soon as I get them 