Their parents give them devices so they don’t have to deal with them. That’s how this slang spreads to them. Do you think 6-10 year olds devoloped “mew?” It was grown ass “influencers” and it spread through media.
You are aware that 6-10 year olds spend time around adults and other kids older than they are right? Did you never pick up anything from an older sibling or kid at your school?