Practice is an acquired skill. You or your child learn to practice by practicing. It takes time to develop the skills and abilities to practice effectively. You or your child start from where you are and develop as your time and awareness allow.
You should not expect yourself or your child to master practice anymore than you would expect to master an instrument within a few short years.
Practice continuously develops and can be enjoyed at all levels along with increasing desire, interest, and ability. The entire process can be a journey of enjoyment and learning with growing confidence.
Encourage the player, not necessarily to practice, but to practice what he/she does well.
Encourage his/her efforts. Practice can develop into a decision of the will, heart, and mind. But this can be lost if practice is forced or used as punishment.
The following is a quote from the book, "Never Too Late," (page 65–bottom) by John Holt, best known for his insight into the way children learn and an explorer of adult learning:
"People who know of my love for music today often say to me, ‘Don’t you wish that your parents had made you take music lessons when you were little?’ The answer is, NO, I don’t. I think that such forced exposure would probably have turned me away from music, as it has so many others." John Holt
Practice is an art form within itself. It takes time to develop and grow with it.