How Can I Separate My Parents’ Apple Devices Connected to the Same Account?

It's pretty clear this setup isn't working out! You’ll definitely need to make new Apple accounts for each of them. Such accounts aren't meant to be shared, and trying to manage without doing this isn't going to solve the problem.

Yep, sharing an Apple account is a recipe for confusion! Everything you're experiencing comes from that setup. Unfortunately, the only real solution without creating new accounts is to go through the settings and uncheck shared services, but they'll just keep getting messages from each other if the accounts are tied together. I really suggest making separate accounts instead.

The main issue is indeed the shared Apple account. All their devices are syncing together because of that. You can either create separate Apple accounts for each of them, which is the easiest fix, or go through various settings like iCloud, Messages, and FaceTime to turn off syncing for the specific devices. But honestly, separating the accounts is the best route if you want to avoid this hassle in the future.

Separating their Apple accounts is the way to go. Just making one account for each of them allows personal management without all this confusion. If they truly don’t want new accounts always keep in mind to check iCloud settings and turn off the ones that shouldn’t be linked.