While Delta should have notified your parents of the schedule change, I’m not sure you will get much more than some goodwill miles for the inconvenience. Not to sound harsh, but this should be a lesson for your parents to always monitor their reservations routinely. Or perhaps have someone who makes their travel arrangements monitor them if that’s the case. And especially so if traveling with a pet in cabin. Those types of reservations have to be constantly monitored to make sure the pet is relisted following every schedule change, and to make sure they are in an appropriate seat that can accommodate a pet carrier.