To be clear, Skull Kid is an assist trophy only, not a new fighter for the game. The Skull Kid character, what a child becomes after getting lost in the Lost Woods, has appeared in multiple Legend of Zelda games including Ocarina of Time and Majora's Mask.
In Smash Bros. games, grabbing an assist trophy summons a Nintendo character who comes to the aid of the player and wields damage to combatants. Sakurai did not provide any details regarding how the Skull Kid assist trophy will work in the new game for Wii U and 3DS.
The new Super Smash Bros. game for Wii U and 3DS will be released in 2014, though Nintendo has not announced a specific release date yet. Cross-platform play between the two versions is not supported.