try again, it was on a maintenance.
click this link and choose the left button with the young kid
you need a young kssn info to be able to get passed the first part of registeration and you need the first adult kssn to verify that you the parent /guardian agrees to let the child play the game.
i think its the company's mistake to force people to swap kssns if the child
kssn is too young to play.
if your kssn's age is in the range of 12~14 you shouldn't get that pop up, you could keep playing that account if you dont mind about crappy sprite quality