From my reading of the report, Israel and the US are the two worst abusers. Boko Haram is a distant third.
Which country is the worst abuser, Israel or the US? Taking into account that the US is responsible for the violence in Afghanistan, Iraq, and Syria that has resulted in torture and detention, if we include these victims in the American total, then the US is the Number One torturer of children. As it is unlikely that Israel could get away with abuse of Palestinian children without Washington's support, we can add Israel's abuses to Washington's total.
Guantanamo Bay is a great distance away from Washington's wars against Muslims in Afghanistan, North Africa and the Middle East. Yet even at Guantanamo, where the only violence is the violence that the US military inflicts on detainees, the US government tortured children, according to the Human Rights Watch report.