Also adding that if you actually buy the game again and create a second service account/subscription, there's no way to actually move existing characters on to it, they'd have to start over.

If you do decide to go down that route with a second subscription (and someone starting over) I suggest creating it on a completely separate Square Enix account. The poster above is absolutely correct that you can have multiple service accounts (up to 8) on the same account so they all share the same login details, and I do this myself, but long term if might cause real problems as those characters will always be on this service account which you are unable to move them to a different account later.

That also means they'll always all share the same login details too which may end up being an issue. I don't know, I'm just pointing this out though. I don't know what SE intentions were with the multiple service accounts, but because you can't move characters/accounts later, it suggests to me they only intended for them to be all service accounts owned/used by a single person, not a family.