Loosing their teeth
The father said to the children that when they either lose their right or left dog teeth, they'd be ready for the dangers in the outside world, and when the tooth would grow back, they'd be ready to drive the car. Since the children did have all their teeth, they believed they were not ready yet.
But doesn't every kid loses their dog teeth at the age of 6/7/8? So, why didn't anyone of the children replied that they already lost their dogtooth and got it grew back and thus were ready to 'go out there'?