Practicing- Students have to practice in order to make progress. If they don't practice, they're going to be unprepared in their lessons and feel unhappy and unsuccessful. Every child is unique and needs a different type and amount of help from parents with their practicing. They may need help remembering to practice, fitting it in around other activities, or sticking with it when they feel tired or frustrated.
This is a great opportunity for you to guide them to learn self-discipline and perseverance. Please don't expect them to manage their practicing totally on their own.
Attendance- Regular attendance at lessons is critical to your child's success. Occasional illnesses and emergencies are a fact of life, but cancelling lessons frequently because of other activities is not wise. It interrupts your child's learning routine and teaches them that you and they are not really committed to their music study.
For the same reasons, cancelling lessons because a child hasn't practiced is not OK.