If they have other traits that match, it'll seem really natural. So maybe make them be introverted, hate loud noises, hate their personal space being invaded, all the things kids do that annoy people. make them especially sensitive to those things? if it's an 8 year old, maybe make the character really insecure and hotheaded, so they're more likely to engage with a kid who's just pushing their buttons
Hating kids is not a sustainable major personality trait ngl. Apathy or distain towards the thought of having kids and refusing to be responsible for another’s kid are grounded aspects of a character. If a kid does something in their vicinity that’s annoying maybe they will just shoot them a dirty look.
If it’s the first, than ignoring the kid when they annoy your character works, excessive scolding, use of threats or generally scaring the kid (“behave or I’ll leave you back where I found you!”), not trying to take care of the kids emotional needs (neglecting the kid, not engaging with them, not checking in when the kid’s upset), making plans without thinking about where the kid’s gonna go
And depending on your character’s personality all of this can b modified. Are they combative? Then they’ll stoop to the kid’s level and taunt the kid, start arguments, make faces, hurl insults. Are they aloof? They’ll just leave the kid alone and avoid them as much as possible, which will emotionally destroy the kid