What happens if you over-discipline or under-discipline your kids?
Over-disciplining or under-disciplining children produces rebels in both cases. The biblical principle of temperance—being temperate in all things—requires maintaining a balanced, middle-of-the-road approach to discipline. You cannot go clear off to the right or left; you must maintain that middle balance to avoid raising rebels.
Les points to verse 25, where everyone striving for mastery is temperate in all things. He explains that being temperate means maintaining a balance—you don't go clear off to the right, you don't go clear off to the left, you maintain a middle balance. He applies this directly to disciplining children: you cannot over-discipline, or you will have a bunch of rebels; you cannot under-discipline, or you will also have a bunch of rebels. Instead, you must maintain that middle-of-the-road temperate idea of discipline. He then compares it to business: a man covered up with inventory won't be in business long, and a man with bare shelves who has to order everything won't be in business long either. So you have to maintain that temperate balance. It's the same way in running the Christian life—don't go off to the deep end to the right or to the left, but maintain that balance, being temperate in all things.