They boy's new bike
The fellow in the store said that technically, Ken would fit a bike with 24" wheels, but I hesitated. The leap from 18" to 20" isn't much, but it's not nothing; the turn radius is larger, the thing requires more starting and stopping effort, it's much longer and requires that he lean forward over the frame. I want him to able to be comfortable and to have to adjust only along one dimension at a time. I reckon that the many upgrades to this stage will be enough.
Plus, the store has a policy of refunding 40% of the purchase price if the bike's returned in good condition within two years. He'll still be eight years old at that time, and much more ready for a 24" at that point. Which, after all, is only 2" shy of an adult's mountain bike.
It is far better to grasp the universe as it really is than to persist in delusion, however satisfying and reassuring.
—Carl Sagan