My son is 6 years old. He's got quite few friends. I like to arrange 1:1 playdates as I think they're less complicated, and it's nice to allow the boys play without anyone being left out, plus I get to know the parent (usually the mum). However, my son has a friend whose mum I am also friends with, and whenever we've arranged a playdate, she'll always end up inviting someone else (without even asking). She'll just mention X is coming too (sometimes the day before). Am I silly to find this frustrating?