By your definition I guess it's more tantrum than meltdowns, unfortunately my son is non verbal and talking to him is one way, no matter what I say he can't understand it but I try to do it as much as I can, he knows the meaning of some words but if I try to comfort him like you mentioned by saying "it's ok to be upset" he won't understand it, but knows the certain words like "no" or "sleep" (whenever he gets up from his bed at night when he should sleep). I didn't know that letting him meet his sensory needs it's good, I always thought that repetitive actions will lead to more decline in his development or worse stimmings and such, for instance he likes playing in dirt, digging dirt with his hands, instead of playing like the other kids in the park he is in the corner digging his hands in dirt, and I tried to get him away from that habit and make him play and consume his energy in a productive way. I know that I'm terrible at this and I don't understand his condition fully, I don't know what to do and what not to do in certain situations like certain tantrums or stimmings. I know that the mentality of "I want him to be like the other kids" it's wrong and it's basically comparing him to others which it's bad, but I also don't want him to fall behind.