I understand the need to practice between lessons. How much time should my child spend practicing?

Practice between lessons is very important and should be encouraged.  Without it, progress will be very slow.  There is no need to practice excessively, especially at the start.  Practice should be enjoyed and not made to feel it is a chore so I would recommend starting with say 10 minutes per day and building up from there as required.  Setting the same time aside for practice works well, so it becomes part of the daily routine.