"Dissociative identity disorder (DID) is a <i>rare condition</i> in which two or more distinct identities, or personality states, are present in—and alternately take control of—an individual."[0]
0. https://www.psychologytoday.com/us/conditions/dissociative-i...
One of the problems is that the kids themselves are so awash in information and misinformation about this stuff, including awareness of how things are defined in the DSM-V, that they can muddy up your attempts at evaluating them.
It's kind of like if you take enough personality tests, like the MBTI, you eventually just sort of know how each question is being evaluated and you can, consciously or unconsciously, start optimizing your responses around the result that lines up with your narrative about yourself but isn't necessarily the most honest set of answers.
That, in turn, is made more difficult by clinical perceptions of MPD: many psychiatrists are openly skeptical that it’s a “real” disorder, meaning that they’re less likely in turn to be empathetic towards a prospective patient who claims (perhaps incorrectly, but perhaps not!) that they have MPD.