Kids in most european countries use whatsapp even though they are under the minimum age.

Ban an app, another appear. Ban all apps and they would join any of the services that provide a web frontend. Kids in the late 90's/early 2000 were using IRC when ICQ and MSN messenger didn't support group chat, usually from a web client before they were introduced to mirc and other irc clients.

Bottom line: they would find a way.

Yes. That's also part of the technical experience that also changes the resulting social landscape. I used to think "what's the point of banning something if people can get it anyway" but after seeing how cannabis became hyper-commercialized in the USA, I see that both the ban and evasion are just part of the game. (Which nobody should get prison for)

There are dozens of ways to have a group chat. iMessage is not enabling this in any meaningful way.

There are, but if kids are using iMessage for it and not using other things even though they could, not having iMessage can serve to insulate a kid from it.

Parental controls may prevent some of the kids from installing third-party messaging apps, or maybe they're just unwilling to. There are a weird number of adults in my social circle who I can't convince to do so, though I'd imagine kids to be a little more flexible.