What the difference between k050 and k124 I have an old kids bike. It has been in our family for at least 30+ years.
However it was left in the rain by the our resisdent bike killer.  Its a bloody mess so unless am told otherwise i think its best to buy a new rear wheel

But am unsure the difference between the two sizes. 
http://www.tredz.co.uk/.Kenda-Kids-12-inch-Tyres_40597.htm
Those are not sizes, they are both the same size. The K050 and K124 are Kenda's designation for tread pattern. 

K124:


K050: