1/2" pedals will fit bikes that have one piece cranks like the picture below. Normally you will only find this style of crank on some childrens bikes and some adult one speed bikes.
A Little Daub will Do Ya
I always put a daub of grease on the pedal threads before installing them.
Junior Pedals VS Adult Pedals
The only difference between Junior pedals and Adult pedals is the size of the pedal. I recommend the smaller junior pedal for most 16" and 18" wheeled bikes. The beginner bikes have the pedals close to the ground so the smaller pedals allow the bikke to lean over further in a corner before the pedal hits the ground.
Left Pedal VS Right Pedal
The left pedal is always reverse threaded and the right pedal is always standard thread. The left pedal will have striations on the pedal axle near the wrench flat that are not on the right pedal. Often the axle of the pedal will have an R or a L designation also.
Check out some Pedals on Ebay
***Before 1990 there were 9/16" English (the current standard), 9/16" French thread (this was on Peugeot, Motobecane and other French bikes, and 9/16" Italian Thread.