I say that with a sense of humor, but the truth is that getting started is one of the biggest barriers to regular practice and, therefore, to progress.
The Germans have a saying, "Aller Anfang ist schwer," which means roughly that all beginnings are difficult. A beginning is a change from one state to a new state, which is hard because, as we may remember from a distant science class, a body at rest stays at rest, and a body in motion stays in motion.
Each time we sit down to practice our instrument, we are beginning something new. We are in a state of transition from not-practicing to practicing, from off to on, from rest to motion.
In a car, the energy needed to go from off to on comes from the battery (and a person turning the key, of course). For a young person, the energy needed to make that transition usually has to come from a parent saying, "It's time to practice" and following up as needed.
Very few kids have the energy to start practicing–to go from rest to motion–on their, but once they start and get past the initial resistance, many kids actually enjoy playing their instrument and working to improve their playing. Even if kids resist practice, they know that improvement is a worthy goal.
So one of a parent's roles is to create the space for practice and to supply the energy to get a child moving. It's not always appreciated at the time, but it's what make things happen.