When Susan berates (who she thinks is) Corin for going off and frightening her, she calls him playmate. Not child, or darling, or dear, or even son. Playmate.
Just. Imagine all the fun they had together! Did they play dress-up and pretend? Did Susan let Corin battle imaginary dragons to save the fair Queen? Did she chase him around the grounds at Anvard in wild, raucous games of tag? Did her skirts fly up and her hair get all windblown? Did he tackle her? I need to know!!