Charles, memoir is one of my favorite genres to read and write and I have several published in magazines. You needn't worry about not remembering things exactly as long as you're not making stuff up. For ex, in Tongue-Tied, I've taken bits of conversations that happened at different times but put it all in one place for the sake of brevity--here's the passage:
My stutter soon became a topic of conversation and speculation.
"Her brain works faster than her mouth," said my mother.
"She has a nervous personality," said a family friend.
"So what? Julius Caesar was also a stutterer," said Mohan.
"M-Moses too!" I piped up.
My suggestion is to write the incident as you remember, fact-check with family members and newspaper accounts and see if it's a strong enough piece to submit to magazines. If you decide it needs more imagination, it will be fiction. I've written many, many short stories with a kernel of truth. You can even decide to write a novel around that incident.
Some helpful books for writing memoir so that you understand it more fully, including its limitations and pitfalls.
Writing a Life by Katherine Bomer
The Memoir Project by Marion Roach Smith
The Art of Memoir by Mary Karr
Old Friend from Far Away by Natalie Goldberg
And of course, read lots of memoirs for kids to see how they've handled it. Some of my favorites:
Anne Frank: the diary of a young girl--I read this when I was 10 yrs old and it began my long interest in Holocaust and Jewish history.
El Deafo by Cece Bell
Brown Girl Dreaming by Jacqueline Woodson
Born a Crime by Trevor Noah (adult but fine for YA)
Happy remembering and writing. It is a great joy to be able to share true stories. Go for it.