Some people care, some people don't. I'm of the "don't care" group. Generally speaking, if part of a family goes to meet up with the rest of their party, they aren't going to be taking spots from anyone behind them anyway. Either Dad would ride in a car alone or he'd ride with his wife and kid. the CMs aren't going to put a stranger next to him on, say PeterPan or Pooh or TSMM, and a stranger isn't going to want to split up from their own family, so if Mom and kid aren't allowed through, then the seats next to Dad just go to waste.
People get WAY too bent out of shape over minor things like this.
Now if it's a group of ten pushing past a huge long line, that's a different kettle of fish, but I'd guess most people would be in agreement that that's not a good idea.