I looked into that one, because it was so outlandish but repeated by multiple sources and reported by reputable journalists.
The original reporting was carefully phrased to make it sound like IDF soldiers were "sniping" little children, taking careful aim and deliberately shooting them in the head, which is... monstrous.
Where this narrative starts to breaks down is that snipers don't aim for the head, because it's too small a target. Additionally, most of the reports mentioned that the children were shot "in the street" in scuffles with ordinary army grunts, so where were these mysterious "child-murdering snipers"? How were they so reliably hitting small moving targets in the head?
It turns out that this phenomenon was a product of Gaza's demographics combined with survivorship bias.
Half of Gaza's population is under 18! Teenagers are technically "children", but they're out in the streets throwing rocks or whatever at IDF troops, and get shot at in return. The ones that get hit by half a dozen rounds to the chest die on the spot and aren't taken to hospital because obviously, hospitals are for people that can be saved, not the dead. All of the reports of "children getting shot in the head" were coming from surgeons in hospitals, but they were seeing a biased sample: victims that only got hit once had a chance to survive. Similarly, a minor head-wound isn't instantly lethal, whereas a single round centre-mass typically is. So they were seeing victims with single shots to the head instead of multiple to the chest. They then cooked up a narrative that would explain what they were seeing, not realising that the cause wasn't "evil child murdering snipers", but simply a statistically biased view of a war that is not especially more unethical than any other armed conflict.
This is likely one of the worst posts ever made on hackernews.
There are videos of Palestinians being shot in the head, yes (NSFL links below)
Here's a video from last year of an elderly man carrying a white flag being shot in the head: https://www.wgbh.org/tv-shows/newshour/clip/war-in-the-holy-... (this news site removed the moment of impact but I'm sure you can still find it)
Here's a video of IDF in the West Bank head-shotting an unarmed elderly man who posed no threat: https://en.wikipedia.org/wiki/Killing_of_David_Ben_Avraham In this one, the man had converted to Judaism after feeling an affinity to the religion due to his grandfather saving the lives of more than 10 Jews during the 1929 Hebron massacre. His Aliyah request was denied, and he was executed while attempting to visit a settlement (by invitation) for Torah study
This one from 2018 was widely circulated (this victim was shot in the leg, although based on the IDF recording here many had assumed this was also a headshot): https://www.theguardian.com/world/2018/apr/10/video-appears-...
Here's a very detailed report on 15 aid workers in Gaza being executed at point blank range and dumped into a mass grave last year: https://www.dropsitenews.com/p/israeli-soldiers-tel-sultan-g...
These are just ones that come to mind as some of the most brutal ones, but there are many, many more videos of the IDF executing unarmed Palestinians who are posing no threat by head-shot. And for every one we see there are likely 4 or 5 more which we don't see because they weren't independently recorded.