Found: YA or younger fantasy/fiction

okay, so I've been trying to find this book for a long time. I remember that was a book series. I think the first book had a red cover with like a gold key? or keyhole on it. The book series is about a group of kids/teens (i think they're with 4) who all live in different cities. Each book takes place in 1 city ( I think the cities are shanghai, London, Paris & NYC?) and they basically have to meet up to save the world against something. something that happens every 4 or 5 years. I think their parents were also like a part of it idk.
I read this between 2009-2013 i think lol.

I know exactly what book you're talking about! I can't remember the title either, sadly. However, I will add that I believe the book had something to do with time travel/alternate universes (unless I'm thinking of something else then please correct me).

Are you maybe looking for the Century Quartet? The first volume is Ring of Fire by Pierdomenico Baccalario, published 2009

>3 vroni: YES YES YES THAT's IT ! I was losing my MIND trying to find this book. THANK YOU SO MUCH

Happy to help!