Ask them to play for you. Don’t ask to hear them practice. No player wants other people to hear them practice. We know that the piece is full of mistakes and probably sounds awful. If a parent just says ,hey, Johnny, play that little piece for me. It sounds kind of cool. I like it. He will probably as which piece you are talking about. You then have to hum a note or two to get him going. The point is to get him to take out the music and start playing. It really does not matter which piece he plays. After that piece is finished, point to another piece and ask him to play it. The most important piece of this time is not to give constructive criticism. Your only job is to get him to play.
How to help your kid practice