I think that there is a danger with kids of assuming that the things they say when they are calm have a greater depth or fidelity or persistence than the things they say when they are upset.
We may like to think that our dispassionate self is more "true" of ourselves, but that doesn't mean it is true of kids. They are always trying ideas on for size, and calm or not, they just don't have the stable view of self and others that adults do.
For instance, one problem that crops up frequently in family therapy is that a skilled therapist can structure the situation so that the dialog between parents and kids will seem more mature and forthright than has ever been true before. It's a breakthrough! The parents are thrilled. For the parents it seems that everything discussed is now solved or dealt with, but for kids, no matter the degree to which they throw themselves into it at the time, it doesn't necessarily stay with them. They are highly responsive to the collaborative environment that was created, but it doesn't mean that they have the self-consistency and self-regulation to make it stick.
Parents find this very frustrating and interpret it as a betrayal. But it's really just that we overgeneralize from the more mature seeming behavior in the session to the assumption that the words have the same meaning that they would in a more mature human. Nah. Still a kid in there.