Let's face it...people don't RSVP anymore and it's rude, people have lost their manners. It is difficult enough to plan a party, but then trying to guess how many will show up makes it that more difficult.
It has taken me a while, but I finally got up enough nerve to say to a non-RSVP guest that shows up "Oh, I wasn't expecting you!"
But I still prepare enough food for everybody I invited, just in case a few people show up with out telling me they will be there.
As for the neighbor's cousins - since you said it was ok for them to come you should provide them with the same party favors you are providing other kids. Again, this is a matter of manners, only your manners this time. If you had told the neighbor "no" then you'd be fine not providing them with party favors if they showed up anyway, but you told them it was ok for them to come, so you need to treat them as an invited guest.
Don't hesitate to show your surprise about people that may show up that didn't respond to the RSVP. RSVPs are requested for a reason and they need to be provided. I'm a firm believer of manners!!!
Good luck on your party and Happy Birthday to your little girl.