Lol wut. The origins of the Spider's little birds are spilled like 3 times in the book. Even once in the very first book. They are normal children, trained special by the Spider's overseas business partners.
Wait the children are the spiders? As in that bald dude in kings landing? Im gonna have to rewatch season 4. It was the first season that I had to wait a damn week between episodes. Its taking all my self control not to just read the books and and find out what happens. Damn HBO, how about a 12 episode season?
Read the books. They are way better.