The policy is that a parent with a child under a certain age cannot sit in the exit row, even if the child is seated with the other parent somewhere else, and even if the parties are on separate reservations. I've flown hundreds of thousands of miles with my (now 9 year old) daughter, and this has been the rule on every airline we have flown for all that time.

It's not a new rule, it's just new to the OP.