Our number one reason is always safety. Lessons are mentally and physically demanding for little ones and we never want to fatigue or exhaust them. For their little bodies this can be quite the workout, so we make sure to limit that time as we want to be sure to see them the next day. No one goes back to the gym after a brutal leg day, am i right?
The second reason is for retention of skills. It is shown that children in this age range attain skills best within the first 10 minutes. We are sure you know this all too well that attention spans for little ones can be quite short.