Well they are really 2 different places. Cali Grill is signature, while Kona is not. They also serve 2 different types of food. Cali Grill is upscale "American" fare and Kona has "island inspired" food. What is your budget? Cali Grill can be VERY expensive, but I think the food quality and view make up for it. While Kona doesn't have a view, you can walk right out to the Poly dock and get a great view of Wishes.
With a 6 year old, and if you're on a budget, I'd say go with Kona. Great food for great price. If you want to splurge, then Cali Grill is way to go. Also, keep in mind that as a signature, Cali Grill has a "dress code" -- e.g, business casual.