How to slow down a kid's bike In the show Yowamushi Pedal, the main character starts out riding a mamachari or "mommy bike" that he has owned since fourth grade. Now he is in high school. In season 1 episode 3, one of the other characters points out to him that in order to go faster he needs to raise his seatpost, and also that his bike has an unusually small front chainring. His bike only has one speed, and because this chainring is so small, he has to pedal at a very high cadence to go fast. The other characters speculate that his mom had the small gear installed for "peace of mind," so that the higher cadence would tire him out and keep him from riding too far away from home. 

My question: The show made it sound like this is commonly done in Japan. Is this done in North America or Europe? And if so, what is it called in English?
This is not common in America.  Perhaps in Europe in places where cycling is more common, but I'm going to say it is going to go the opposite way:  going to higher gearing to get the kid going faster, or even to go to lower gearing to if they have trouble getting them going.

As for America, we don't really buy kids bikes with specific gearing.  It is something that isn't advertised for the bike (unless that is the major selling point of the bike).  Many of the 1 piece cranks we have are not capable of swapping chain rings.  However, the rear hubs still use the common 3 spline interface for single speed cogs.  I have swapped out the rear cog on a kids bike that we used for mountain bike trails because the gearing was too high for the kid.  However, despite the popularity of mountain biking in my are, my kids seem to be the only ones doing it on a 16" bike.