I have a son in the same age range as your son and we go to Oahu frequently so here are some suggestions for you, based on our experience....
If this is a once in a lifetime trip, I'd do Pearl Harbor. However, if you're going to come back, I'd skip Pearl Harbor. Although historical, it's boring and hard to understand for kids of that age group. They don't understand what's going on....
And as far as diamond head goes, the hike is fairly mild, however, if your kids don't like to walk, or do the whole, my feet hurt, thing, this would not be the activity for you.
I would recommend the Polynesian culture center and the luau there. It's very fun and educational and most kids really seem to enjoy it. That will take up all of your day. I would also recommend driving aroud the island. If you want, stop at the Dole plantation during your island drive, but I would not make a special trip just to see that. My son also loves going on the Catamarran boats. You can catch them in front of Outrigger and they're usually about $15/person for a 2 hour ride. Depending on the time of the year you go, you can see dolphins out there.
I would also walk through the international market place and let our kids pick out their souveniers. They have lots of fun little knick-knacks.
We tried snorkeling with my son last year when he was 6, and he couldn't get the hang of keeping the snorkle upright and breathing through it. I don't know if that's normal for that age(our tour guide said it was) but that was our experience with that.
When planning, keep in mind that you're in Hawaii. You don't need to plan a whole bunch of things to do. The kids will be perfectly content playing in the sand and swimming in the ocean. Also, there are little beach shacks in front of Hilton Hawaiian Village (HHV) that rent the big ocean bikes, kayaks, and a few other big things to take in the ocean. They're very, very reasonable, so it may be fun to rent one of those and explore. My son loves the bike one and you don't have to peddle out very far to see fish swimming in the water. And the HHV has several nice pools. Let the kids play in the pool while you relax and have a drink
Also take the time to explore the HHV. There's lots of animals: birds, fish, ducks, penguins, parrots. And a lot of shops and restaurants. Also, on Friday nights at around 7-7:30 HHV does a fireworks on the beach display and does fireworks. Have a wonderful trip! You and the kids are going to love the HHV! It's a beautiful resort.