When is the best time to practice my music (with my child)?
The short answer: Make sure that it is a time when the musician ( or in some instances parent and child ) are awake and ready to concentrate.
Many people are very concerned with how long to practice but they do not give much thought to the time of their practice. Some musicians, and parents, think you can just shove the practice somewhere in the day and get it done. But this only breeds frustration, slow progress, less enjoyment and may even pick up some bad habits when there is tiredness and distractions for most people.
For young children, the best way to do it is to have the same practice time throughout the week. As William and Constance Starr commented, "They grow to expect practice every day at the same time. Practice then takes on the quality of inevitability. It becomes part of the routine." For example, I have seen parents that practice with their children every day before or after dinner. Other musicians practice during homework time, as a kind of break from the academic work. Still others wake up early and do it before leaving for school. If that does not work, try to have two practice times available to alternate throughout the week. It takes time for children to get used to a more flexible schedule, until then structure is the key to success. Having a scheduled lesson makes things easier for everyone as the child has energy and concentration and is already expecting to do it.
For those of you in high school, college and beyond, find a time that gives you the possibility to make the most of your practice. You do not want to have distractions around or be exhausted from work. You have to be honest with yourself about what time is best.
Remember, practicing is not just a chore to rush through. You want to be able to have fun, enjoy yourself, get through challenging parts of a piece, and many other things. The more you make the most out of your practice time, the better life will be. So, ask yourself: "What am I practicing for?" The answer to this question, the motives that you have, is what will drive your practicing approach.
This is nothing new, but I hope this reminder has helped you realize that it's worth it, keep going!