Perhaps it wasn't a sniper round. Perhaps it clipped their head. Perhaps the parents wanted to try to save their child anyway. There's many reasons why this is plausible.
You are looking for explanations as to why the story you like is true instead of examining it critically. It is like those who believe in a literal interpretation of the Bible and believe an absolutely not plausible story of the Noah Ark. They will look for ways of how it could actually happen. It is pointless to point at geology, biology, geography, physics, and other facts that contradict the story
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This story may be based on reality. It is very likely that a group of children was brought to a hospital and some of them head wounds in their heads and no one of them survived.
But why create a story that tells a dramatic cinematic propaganda story? Because the goal is not reporting, goal is to extract emotions and paint Israelis as demons.
It is a rather obvious fake staged propaganda crap.
Does it mean that there are no injured people in Gaza? Nope. Thousands there have far worse injuries. But this is an emotion-inducing propaganda piece. A young injured Palestinian in a dramatic pose looking defiantly into the camera offers a cool and cinematic retweetable story.
This children murdered by snipers story is a similar BS designed to be a cool drama. There are many tragic events happening in Gaza. There is a healthy dose of actual Israeli war crimes.
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Edit to illustrate
A Palestinian father and daughter were killed by Israeli snipers in Gaza
https://www.youtube.com/watch?v=6__JTz_1iso&rco=1
Here, we see a blurry video of dead people accompanied by someone's words. As we can see, for many people, it is enough of proof of intentional Israeli sniper fire, as if there are no other possible causes of death in a war zone.
What can I say to people who eat propaganda so happily and readily? Really, nothing