The length of a child's practice session is always a fun topic among parents, students and teachers. I personally hate timed practice sessions and would encourage parents everywhere to throw away the timers.
Telling your child to "sit down and practice until the timer goes off" is like sending them to their room and telling them to "clean until the timer goes off." Would their room every really get clean? I've found the more specific I am with my children about their chores, the better the end result is. Practicing is the same. Make your sessions task oriented. I like practice sheets as do my children as they know that when they've checked off the "to do list" the session is over!
Here is an example:
Warm up- 2 minutes of something very easy to get their fingers going. This should not be something new. Warming up is essential...much like stretching before you go out for a run.
Review - This should be fun and could be considered part of the warm up. Pick a piece that they already know and like. Reviewing old pieces not only helps memorization and retention skills but it is the perfect place to work on technique as it is near impossible to have perfect technique when plunking through a new piece.
New Piece - You should really only work on one new piece per session. Any more than this can get overwhelming. More on how to tackle new pieces later...
That is it ~nice and concise!