So this is what you bring me? Anyways him being stronger is not the point here, as he is even capable of being stabed by a blade. There's nothing that would even prove that Killua couldn't stab him with his hand, plus with lightning. Being stronger than someone doesn't win a battle it's all about abilities. Killua out beats Zoro in this. He's faster, which is a huge factor for this battle.
Now Ownagepants has a good point. It's in Zoro's character to put his guard down when it's a child. But I'm not arguing this factor, I'm only looking at the fact Killua is capable of ripping his heart out of his body.
Zoro has tanked piercing attacks that collapse buildings while half dead in earlier arcs, while Killua's sharpened hand has been stopped by abdominal muscles of a character far weaker than him, strength does have relevance here. But for arguments sake lets say that Killua does have the ability to easily pierce and pull out Zoro's heart, given Zoro's reaction time and his showings while fighting faster characters it isn't likely that Killua will be able to blitz him even with Godspeed, outpace him sure, get close and land a lethal blow, unlikely, he has a better chance at a distance using his weighted yoyos.
Zoro may be taken off guard because Killua is a child and subsequently lose, but whats the fun in arguing that?