the last time you saw a child, horrifically maimed, in a Hollywood movie?

There are no rules against it, it's just that most filmmakers consider it in poor taste to show a child graphically murdered. But I think it really depends on the tone in which its presented. Sometimes it's for pure shock (such as a girl getting her face blown off in the Lucio Fulci flick The Beyond). Sometimes it's dark humor (like in the movie Bloodletting, where in the middle of a murder spree, a killer is startled by and inadvertently shotguns a baby in its crib). And sometimes it's just a tasteless and gratuitous attempt to anger or provoke the viewer (such as the infant rape in A Serbian Film).

As for the last time I saw a child graphically maimed in a movie, I think it was the season 5 episode of The Walking Dead when they're trying to sneak through a horde that's flooded Alexandria, when one kid panics and the walkers attack. I only saw the episode once but I seem to recall at least one of the kids get graphically bit.

IT

Hereditary (new movie)

Not maimed, but see the ice cream truck scene in the original "Assault on Precinct 13" (way back when in 1976).

That movie out now has a horrific child injury. Not to mention the Exorcist. It's not forbidden.

mother!

Though I didn't watch it until after some of the other films mentioned (Logan, It).

In the 2017 film "Logan", the young girl Laura got shot through the chest with a harpoon. She was actually survived because she had a healing factor. https://www.youtube.com/watch?v=xypetVQB8fs (4:10 mark)

In the 2017 film "IT", a young boy Georgie had his armed ripped off and he was shown crying and crawling away to escape the monster. https://www.youtube.com/watch?v=TAWVC71nzyA (3:24 mark)

No such restriction exists.

The tv series Walking Dead has had some children who are walkers. In one episode, a child is shot (off camera, but it's clear what happened).

I can't think of any movies off hand.