You can log in with your account on multiple devices (including across iOS and Android) and the progress and subscription will be carried over. This also includes when using Multiple Profiles. Each member of the family 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.
Just be sure you check the My Account page in the app menu or the first device so you log in with the email you used there. Because of this, you do not need to enable Family Sharing or Family Library for Simply Piano.
However, if for some reason, your subscription disconnects from your account, you will need to Restore Purchases with these directions.
Have more questions? You can talk to us through the app. Tap on Menu > Settings > Send Us Feedback / Contact Support :)