I don't honestly see Gon & Killua having any business fighting the king. Fighting 2vs1 with a captain I could see them winning, with the correct win or die trying attitude. However, I deem to face a royal guard they would first need to go through some shounen training gaining lots of new power. I'd prefer the king to be defeated by someone else as it it would feel cheap if Gon and Killua could pull it off based on what we saw in this episode. In the end those two are still brats and it was only a very little time ago that they discovered nen. Gon is still a far cry from Uvogin. So, they don't need to be able to save the world just yet.