Narcissa!! She's such a chameleon for me -- I can imagine her so many ways. I'm going to answer this with my autistic lesbian Narcissa where Andromeda is very close in age to Bellatrix, and Narcissa is a few years young.
16. A childhood headcanon
18. How do you think they were as a kid? (Like, were they shy, noisy, wild, etc)
So it all absolutely depends for me on what her age gap with her sisters is BUT she was an absolute brat in both a charming and understandable way. She was both her mother's favorite (blonde like her! wants her approval in a way Bellatrix and Andromeda never did!) and also least favorite (the final failure) which really does a number on her emotionally. She has a very hard switch from "I am a princess and I have a hard time thinking outside of my own needs" to "I will carry this damn family on my back because no one else is." Interestingly, that's when her grades really drop in school because she reprioritizes to family and finding a rich husband when she realizes the family finances are actually shakier than the Black family pretended they were.
Mirrorball by Taylor Swift
She also had sensory meltdowns as a kid which was part of the reason she was labeled bratty and difficult, but she learned to box up those feelings and self-soothe alone and also to use various charms to reduce light and noise and change the feel of her clothes.
She learned how to perform in all of her social interactions to the point where she forgets that it's possible for her existence to be anything other than performance. (She could drop the performance for her sisters and cousins. Sometimes.)
Ever since she learned other people outside her family exist, she finds the idea that she's anything other than superior to others based on her birth to be comical
She hated Sirius at first because he stole the attention. Then she hated Sirius because she worked so hard to be perfect, to deny her own needs or to meet them on her own, and there he was... just existing authentically despite all of the consequences. (She was so, so very jealous.) (She loved him.) (She always tried to protect him.) (She loved spending time with the family tree and had it all memorized by the time she left for Hogwarts.) (She stared at the burn marks for hours.)
Childhood headcanon - she had a very honest "obviously girls are prettier so I only want to kiss girls" thing as a child. The switch from "aww this is adorable" from adults to "that's not acceptable" was hard and confusing, but she was a quick learner. She never said something of the sort again.