Chapters have been as short as a single sentence; Stephen King is famous for a single sentence chapter. Other famous authors have written chapters of a single page.
In general, a chapter break occurs when the story changes POV, or there has been a space jump or time jump AND a change of cast. We change chapters for the same reason a stage play closes the curtains and re-opens them on different scene.
If there is JUST a time and/or space jump; you don't have to change chapters; you can cover the transition in a paragraph. Bob and Charlie are in Seattle, we use a paragraph cover the uninteresting travel part of the story, then we are in Miami. For example:
Eight hours later, they landed in Miami. They took a taxi from the airport to the Greenpoint Hotel, and found their room on the fifth floor.
"You take the left bed," Charlie said. "I can't sleep with somebody to my right."
"Yeah sure," Bob said. "We going out for dinner? You want to order something in? They got this list of places."
He held up a colorful brochure of restaurants that delivered to the hotel.
"Nah. We'll ask the front desk, find us a Chinese place we can walk to. After all that sitting I got to pump some blood and get my legs right again."
Done! The transition is that two sentence paragraph; The reader is beside them in Miami, we sneak in something about the uncomfortable nature of the ride, Bob and Charlie are firmly in the Greenpoint in Miami. (Which may or may not exist, I just made up this little story for this example).
But YOUR chapter probably makes sense as you have it, make it six pages without the back story.
That said, it is likely a different mistake to kill the kid so soon. A story generally opens on "the normal world" for the MC; how life is for them before it changes. The change here (a sibling dying) should NOT occur in the first chapter, it should occur in the 2nd or 3rd.
So I think you are probably rushing your story. It is The Normal World that introduces our MC, perhaps the family and the doomed boy. Killing somebody the reader does not know, to affect somebody the reader does not know, does not affect or engage the reader much at all. You are giving them all strangers, saying some stranger died, and expecting that to have some emotional impact on the reader.
It will not. Readers must be introduced to the character first, and THAT is what Chapter 1 is for. Whomever your MC (main character) is, tell some of this backstory in Chapter 1 through their eyes, how they felt, where they left, and in particular interactions the MC has with their brother, the doomed boy. Make the doomed boy sympathetic: Fun to play with, innocent, whatever, a typical kid having reckless fun, looking to the future, excited to be in a new place, going to a new school, meeting new neighbors and friends already, until you put him in the blender. THEN his death means something, to the MC and his parents that loved him.
Typically in a story, the "Inciting Event" that the story is about will occur 5% to 10% of the way through the story. (Read up on the Three Act Structure). If you are in a hurry, you still need about 5% of "Normal World" before you do something drastic. In a 300 page novel, with 250 words per page, that is 15 pages. I'd suggest the following structure:
Chapter 1, The Normal World. For you, the family in transition, getting settled, particularly for the MC and the doomed boy. May be short; 2% of the whole story.
Chapter 2, time jump to a more settled Normal World, perhaps months later, kids in school with friends. Pick an event: A birthday, fourth of July, last day of school, whatever. At the end of Chapter 2, kill the boy; that is the final scene of this chapter and the "inciting incident". This can be 3% or so of the story.
Chapter 3, time jump to "the new world", perhaps leaving the funeral, or days after the funeral, with the family still grieving; that would be the natural aftermath.