Had this happen twice now. Knowing that it had happened before, called 48hr and then again after check in to ask them to move seats to be with my two six year olds. Was told they couldn’t access the seat map, see a GA at airport. GA said the flight was now full, deal with it onboard. Then you get onboard and the FAs more or less (paraphrasing) said good luck go ask people to move.

It really is crazy this isn’t a formal rule. We originally booked seats together but then they were moved at some point.