Newt, the 12 year old girl introduced in Alien, is killed off screen. Her body has an autopsy performed on it to see if her death was caused by a Xenomorph, but most of it isn't shown, with the camera mainly focused on Ripley's reaction.
There is a dead baby which we never see alive, nor do we see the murder. It is only a ghost baby.
The main character's sister and another teenage girl are killed.
Gone With The Wind
The protagonist's small child is thrown from her pony and breaks her neck.
I Am Legend
There was a helicopter and it went BOOM and the kid in the helicopter was like AAAHHHHH and Will Smith was like NNNNOOOOOO.
Noroi: The Curse
Kana Yano dies. Junko Ishii's neighbor and her daughter die in a car accident.
Rogue One: A Star Wars Story
It is not shown directly, but a child is saved from the middle of a guerrilla war zone in Jedha City, only for the entire city to be destroyed shortly after by the Death Star.
South Park: Bigger Longer & Uncut
Suffers severe burns when he attempts to light his fart on fire, but is ultimately killed by the ER doctors who replace his heart with a baked potato, which explodes and splatters some of his remains across the hospital room.
The beginning of the movie the vampire kills a family and a baby.
The Girl With All the Gifts
One kid beats another kid to death with a baseball bat. In fairness, the second kid wanted to eat the first kid's mom.
The Witch (2016)
Almost everyone in the movie dies, including a baby and two small, yet obnoxious, children.