Children are very responsive to and are made very anxious and sad by parents who are speaking negatively on any topic. To talk about the other parent negatively, we would have to think about the level to which this is typical in intact families. Every spouse occasionally says something negative about the other spouse, such as, “Your dad left his dirty socks in the living room again.”
Before we could actually say what the long-term impact was, particularly of talking negatively about the other partner, we would have to have some sort of baseline measure of how often people do this in any case. That would allow us to say what is really constant negative talk. Until we have that, we can't answer your question, except in a general way. Hearing a lot of negative talk is disturbing to anyone, to children as well as to adults.