Have you ever wondered if there was a Piltovan kid who likes hanging out with Zaunite kids?

Probably not uncommon actually. Though class differences probably lead to such friendships becoming short lived.

It's a Romeo and Juliet tale waiting to happen. Or a gang war.

They are probably like the sneeches (I think that's how you spell it) where the parents won't let this happen.

Aren't there areas where both cities mingle? Could've sworn the Progress Day story (before we knew the ending) had kids from different cities.

Jinx and Vi is still mysterious though... only them that have relation to both...

Out of the champions, I'd assume the reworked Orianna lore might suggest that at one point she may have been friends with kids from Zaun, despite having been born, and raised in Piltover. Although who she would've been friends with is the question, it's a possibility that you might be interested in.