Check out this article by Anthony Mazzocchi:
There are a lot of solid ideas in this article. Like the author, I like to use something to help count repetitions; my students like to use buckeyes instead of pennies because we like the way the sound when they are rolled on the music stand. In the fall, buckeyes are all over the front yard and students love helping me gather them.
I think most children are able to practice longer than the guidelines suggested in this article. Ten minutes is a good goal for the earliest beginners, but I think we can expect more. My own kids (age 8 and 10) do one practice session in the morning before school and one in the afternoon before dinner. Son, age 10, works between 45 minutes and one hour every day. Daughter, age 8, practices about 30-40 minutes. Practicing is part of our daily routine, not something we try to cram in before bedtime.
My husband, a pianist, and I have lively discussions about how long the children should practice. Being a flute player, I think it’s easier to practice the piano for longer. The flute is a very physical instrument. I challenge anyone to breathe deeply and forcefully while holding their arms parallel to the floor for 30 minutes and not feel tired.
In my experience I have found that kids HATE practicing slowly. It’s the # 1 thing we parents and teachers repeat to students, but it’s really, really hard for them. Goodness knows my children won’t do anything slowly. They run everywhere and shovel food in their mouths as fast as possible. I think it’s good practice for them to try to play slowly, but it’s unrealistic to expect them to do it consistently.
Gentle reminders, being present in the room while they practice, and making a game out of “being slow” are all helpful, but most of the time young students are going to play fast. Of course slow practice will yield good results, but their little brains crave entertainment. Playing tunes at snail speed is not much fun. My suggestion is to keep a balanced approach by offering encouragement but don’t turn this issue into a power struggle!
Adult students seem to gain even greater benefit from slow practice, but our brains and bodies seem to be a little slower too.