Twice per week is a good starting point. Here’s why: if your child attends only once per week he will tend to fall behind the other students in class. It is difficult to make progress only training once per week unless your child invests extra time during the week to train. This is difficult if you are not skilled in the martial arts yourself.
If, on the other hand, your child attends classes virtually every day of the week, like some teams sports require, he or she can suffer from overtraining injuries and not be able to enjoy other activities.
2 days per week is ideal to ensure steady progress and prevent burnout. It also allows your child to participate in other worthwhile lessons, like music classes AND have unstructured play time, to just be a kid. In this way your child’s martial arts and lifeskills training easily integrates with the rest of his schedule.