An earlier question I posted was about zombie children. However this is more annoying. How do Pigmen children become pigmen children? How can I avoid this from happening?
Pigmen children become pigmen children in the same way adult pigmen become adult pigmen - they randomly spawn in the Nether, and in Nether Portals in Overworld.
You can prevent pigmen children becoming pigmen children if you prevent adult pigmen from becoming adult pigmen.
As light level does not prevent zombie pigmen from becoming zombie pigmen, you have to use slabs, stairs, or other blocks that do not let mobs become mobs.