In 'The Chronicles of Narnia' what part of the old rhyme suggests that the children will help to save the Kingdom of Narnia?

I'm not sure there is a rhyme, but Mr. Beaver says, ". . . it's a saying in Narnia time out of mind that when two Sons of Adam and two Daughters of Eve sit in those four thrones, then it will be ththe end not only of the White Witch's reign but of her life, . . "