Short answers, pulled from the story.
Estimates range from 200,000 to 500,000 Serbs killed by the Ustaše regime in the NDH between 1941 and 1945. Serb statistician Bogoljub Kočović estimated 370,000 total Serb deaths (combatants and civilians), while Croat demographer Vladimir Žerjavić put civilian Serb deaths at 197,000. Historian Rory Yeomans stated the actual number may be as high as 500,000.
Jasenovac was the largest extermination camp in the Balkans, operating as part of the Jasenovac-Stara Gradiška complex in the NDH. An estimated 100,000 inmates perished there, the majority Serbs. The Ciglana sub-camp had a mortality rate of 88%, higher than Auschwitz's 84.6%, and victims were killed through starvation, disease, and extreme physical violence rather than primarily through gas.
The Ustaše was founded by journalist Gustav Perčec and lawyer Ante Pavelić in late 1929 or early 1930. Pavelić became the supreme leader, giving himself the title Poglavnik (equivalent to Führer) when he declared a government in Zagreb on the 16th of April 1941. The movement was financially and ideologically supported by Benito Mussolini.
The Catholic Church in Croatia, headed by Archbishop Aloysius Stepinac, greeted the NDH's May 1941 law on forced conversions and incorporated it into Church internal law. On the 6th of February 1942, Pope Pius XII privately received 206 uniformed Ustaše members and blessed them. The bishops' conference in Zagreb in November 1941 declined to condemn the forced conversions or the killings of Serbs and Jews. Stepinac was later convicted of forced conversion after the war.
Yes, the NDH was the only Axis satellite state to establish concentration camps specifically for children. Special camps for children were set up at Sisak, Jastrebarsko and Đakovo, while Stara Gradiška held thousands of children and women. At the Sisak camp alone, 6,693 children passed through, and between 1,152 and 1,630 died there.
The post-war Yugoslav communist government under Tito suppressed independent investigation out of concern that accounting for the genocide would inflame Serb-Croat ethnic tensions and destabilize the new communist regime. Tito passed Jasenovac several times but never visited the site. Historians Marko Attila Hoare and Mark Biondich later wrote that Western historians have not paid sufficient attention to the Ustaše genocide, and several scholars have described it as a lesser-known genocide.