Lol imagine thinking you are an expert and then saying a diagnosis of Borderline Personality Disorder in children is often helpful.
The description in the DSM of a person with borderline personality disorder is the caricature of a toxic narcissist, stupid, impulsive, irrational and self-absorbed with no redeeming qualities.
Of course children can often be impulsive, display "inappropriate" anger, fear abandonment, have intense interpersonal relationships, and display affective instability, but that's probably because they are *children* not because they have "borderline personality disorder".