Great news! You can log in with your account on multiple devices (including across iOS and Android) and the profiles, progress and subscription will be carried over. To be sure you log in with the right email, check under Menu > Settings > My Account on a device you're already logged into. You do not need to enable Family Sharing or Family Library for Simply Piano.
Each member of the family that has a profile needs to use the same email account to log in and then press Menu > Switch Profiles to access their profile. The account can be used on multiple devices at the same time so different profiles can progress at the same time.
Have more questions? You can talk to us through the app. Tap on Menu > Settings > Have a Question? / Contact Support :)