There is no shortage of food to be had in Waikiki. There is the famous Perrys Smorgy on Kuhio Drive that has a different buffet for breakfast, lunch and dinner and is said to be good once on a trip but twice may be a bit much. There is everything from low end fast food to some of the best restaurants in the country.
Traveling with kids means that you may have to eat some fast food although I do know of a family whos kids got sick on a trip from eating nothing but McDonalds. The best bet is to vary your meals and try to eat like you do at home.
First of all try to eat a great meal for breakfast. We found that the IHOP on Kuhio was a little too much food for the kids and if you can not bring it back to the room for later or tomorrow then the food is mostly wasted. Actually the best meal to eat out is dinner because if you have a lot of leftovers it is easy enough to bring back to the hotel and huck it into the fridge.
Lunch for our trip varied a lot. If you are at the beach it is easy enough to find a hot dog and hamburger but on some days when we got out really early the kids were drained and wanted to get back into the hotel room for a nap and some quiet time by the pool.
Finally dinner is the one meal where you can tend to splurge, if the weather cools down a bit the kids will want to eat and whatever they do not eat can go in the fridge to be eaten later in the evening or at lunch the next day. I have written about why you should always have a kitchenette when you travel before and this just emphasizes it.
Lastly to get food for the room it is a good idea to hit the Walmart by Ala Moana mall or just go to the grocery store at Ala Moana mall. prices there for food and even souvenirs are much better then anywhere in the middle of Waikikis hotel strip.