I noticed the father (some guy named “Thunder” McHaffie) made sure that from the very beginning, this child is making it a habit to wear a helmet and gloves if he wants to ride.
The model that was chosen is the smallest Kuberg, called the “Start”. The tires are listed as 12.5 X 2.4, so it sounds like the wheel rim diameter is about 12 inches.
The brushes in these common motors will wear out after a couple of years of steady use, but they are cheap and easy to replace (the brushes are the gray blocks near the center). These 24V motors have been run at 36V, but that would require a new controller and a 36V battery pack, like these 36V lithium cordless drill packs from Bosch.