Melbourne-born Prakash, who was linked to several Australia-based attack plans and calls for lone-wolf attacks against the US, was killed in an airstrike in Mosul.
The Fijian-Indian-origin, a senior Islamic State recruiter and terror plot instigator, had been arrested in an unnamed Middle Eastern country.
The 15-year-old, who was not identified, will serve at least five years in jail for inciting terrorism and will not be released until he is considered not to be dangerous.