The daily reminder
From a child’s card, you can enable a reminder and choose its time. It only fires on days without a session: a child who has already practised is not disturbed. A reminder that goes off every evening, even after a session, ends up being switched off.

Notifications must be allowed for the reminder to work: on first activation, the device asks for permission — accept it. If it was refused, go to Settings › Notifications › Flash Vocal — Subtraction to allow it, then come back and enable the reminder.
The reminder is a local notification scheduled by the app on the device: no server is involved.