How to slow down a kid's bike
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.