Looking to write the prologue to my story as a 6 year old's POV. Don't want to give away too much, but it's the middle of the night, the house is a mess, he goes up to his parent's room and finds his dad killing his mother. Already acquainted with the concepts of death and murder.
What's the question exactly?. Are you asking how to write it, or are you asking if that idea would work as a prologue?. If it's the second, as usual, it comes down to asking yourself if that background story is relevant to the book's main story, or, at the very least, if it's relevant to the MC's personality or decisions in the plot. If their mother's murder is a pivotal point of their psicological development, or it affected their outlook in regards to relationships, or the nature of humanity, then it absolutely works just fine as a prologue, at least in my view. I defend prologues as a way to give the reader a hint about what the book's themes will be, or to give some background that explains something important about either the MC, or the antagonist, so I would say go for it. If, once it's done, you judge that it's not useful for the reader to know the details of that story, or that it has a completely different tone compared to the first chapter, then you can just consider scrapping the prologue. Hell, I've written three prologues for my current novel, and only the third one is really convincing to me, and it's the shortest of them, not even a full page.
The only advice it comes to my mind about a prologue on such POV is to keep a really simple language, without notorious rhetorical elements, for credibility's sake. Otherwise, to portrait events on such POV is incredibly difficult, being an issue even for child related professionals to handle properly, specially when dealing with time-related elements of the narrative. Unless there is a previous research about child psychology, the risk of sound really out of place is great... maybe an "interview" format on flashback would be easier to achieve, since the needed information could be delivered with an adult structure that most of six year old kids lacks. But it's just a suggestion, of course.
Keep word choice and any dialogue the kid is speaking simple. Also, little kids don't understand a lot of bad stuff in the world. You might consider having the kid not really understanding the concept of death and that once someone is dead they aren't coming back.
As mentioned by the other users in this thread, keep the language and word choice simple. Kids see the world differently compared teens and adults. I would suggest reading passages written by kids to get an idea how they think, perceive, etc.
My thought on this would be to imagine a professional counsellor has asked the 6yo to write down their experiences and thoughts on paper. Then imagine you are reading the child's scribbles, complete with spelling and grammar mistakes and possibly have some mistakes crossed out and re-written. You may already have done some research of course, but this link may be helpful: https://www.cruse.org.uk/Children/children-understanding-death#fiveyears
I'd just write in my usual style but try to keep the language and concepts simple, and double-check his interpretation of events against cognitive milestones typical for his age.