How much children were killed by Nazis in World War 2?

During the Holocaust, children were especially vulnerable to death under the Nazi regime. According to estimations, 1.5 million children, nearly all Jewish, were murdered during the Holocaust, either directly or as a direct consequence of Nazi actions.

The Nazis advocated killing children of unwanted or "dangerous" in accordance with their ideological views, either as part of the Nazi idea of the racial struggle or as a measure of preventive security. The Nazis particularly targeted Jewish children, but also targeted ethnically Polish children and Romani (also called Gypsy) children along with children with mental or physical defects (disabled children). The Nazis and their collaborators killed children both for these ideological reasons and in retaliation for real or alleged partisan attacks.[1] Early killings were encouraged by the Nazis in Aktion T4, where children with disabilities were gassed using carbon monoxide, starved to death, given phenol injections to the heart, or hanged.

1,500,000 children, nearly all Jewish, were killed by the Nazis. A much smaller number were saved. Some simply survived, often in a ghetto, occasionally in a concentration camp. Some were saved in various programs like the Kindertransport and the One Thousand Children, in both of which children fled their homeland. Other children were saved by becoming Hidden Children. During and even before the war many vulnerable children were rescued by Œuvre de Secours aux Enfants (OSE).

https://en.wikipedia.org/wiki/Children_in_the_Holocaust

On October 10, 1944, 800 Romani children, including more than a hundred boys between 9 and 14 years old, are systematically murdered.

Auschwitz was really a group of camps, designated I, II, and III. There were also 40 smaller “satellite” camps. It was at Auschwitz II, at Birkenau, established in October 1941, that the SS created a complex, monstrously orchestrated killing ground: 300 prison barracks; four “bathhouses,” in which prisoners were gassed; corpse cellars; and cremating ovens. Thousands of prisoners were also used as fodder for medical experiments, overseen and performed by the camp doctor, Josef Mengele (“the Angel of Death”).

https://www.history.com/this-day-in-history/eight-hundred-children-are-gassed-to-death-at-auschwitz

During the Holocaust, children were especially vulnerable to death under the Nazi regime...
Although they are the lesser known Nazi atrocities, it is estimated that some 5000-8000 children with physical and intellectual disabilities were killed in Nazi Germany under a programme of euthanasia. Chronologically, they were a precedent, being the Nazis' first organised and systematic killing programme that would later enlarge to include adults with disabilities and ultimately, to the broader programme of racially motivated 'euthanasia' of the holocaust.

https://pubmed.ncbi.nlm.nih.gov/21843187/