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.