As often as not, that works the other way. Folks do not send the RSVP but just show up. This is such basic etiquette, hard to imagine folks can’t take a minute either to respond or to call to say they can’t make it after all. Terrible manners. I really don’t want anyone who’s that rude as a friend.
Yes, the kid should have called. But we are talking about kids, right?
I can think of half a dozen reasons why this happened. Most of them include a husband telling the party host their kid will show up...and then forgetting to tell the mom.
Not like I speak from experience or anything.