In the light of recent canon, I think a lot of Fay's attitude and childishness comes from the fact that he's never been ABLE to, before, in a "safe" enviroment -- I don't buy that Ashura was a horrible person, but he seems to be very stern in his kindness, and knowing Fay, he probably tried very hard to appear adult/competent to show his gratitude to his king. But with Kuro-rin and the kids, they don't have expectations of him, and he can totally cut loose. :D

I do want to write more KuroFay! I just, uh, need to figure out what. And also, I am very easily bribed. :3