Someone will surely correct me if I'm wrong, but I believe scimunito is related to scemo, which once upon the time had the quasi medical meaning that "idiot" once had in English (that is, it was a diagnosis).

You're right to be concerned about register, but you may want to be guided in your choice by whether the boys who make the comment are referring to a physical disability or a mental one (or simply don't make any distinction). "Scimunito" in the context of a physical handicap seems odd to me.

You'll find many more synonyms that you want at urbandictionary.com, if the use is contemporary. If not, I'd stick with the terms I've offered or reasonably equivalents.