Add me to those saying that they would have trouble hiding their annoyance. Weddings are expensive - everyone knows that. And most places charge full price for kids' meals, too. It isn't $10 for some chicken nuggets!
Call me paranoid, but I really, really doubt two little boys would express excitement about a stranger's wedding, unless SpongeBob was the emcee or something. I suspect Nellie knew very well they were not invited and was trying to set up the embarrassed 'oh, yes, um, yes, of course they can come...' reply that is characteristic of those taken off guard.