I think it comes from the word meltdown like as in a nuclear plant going though a meltdown. But for humans when we go through one, we get overloaded, can't handle our own feelings, can't handle the sensory overload, we get so overwhelmed, so we meltdown. Of course everyone has meltdowns and so do children but we are more prone to it.
Just my guess anyway why we call it a meltdown.
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Son: Diagnosed w/anxiety and ADHD. Also academic delayed and ASD lv 1.
Daughter: NT, no diagnoses. Possibly OCD. Is very private about herself.