Brains are networks.
Our concept of a unified "self" is illusory
and collapses when subjected to any serious philosophical scrutiny.
If some people prefer to conceptualize themselves as multiple entities sharing a body,
that's no less accurate than the useful fiction
that, for instance, we remain the same person throughout our lives.
I agree that most people who identify this way
can't accurately be described as having a mental disorder
-- because there's nothing wrong with it.
It's true that you shouldn't revel in your mental dysfunction
or let your identity distract you from your real problems.
However, there's no reason why people can't have a plural identity and choose not do those thing.
The kids are alright.