Hi Claire,
This is a tough one to answer. I really like California Grill and you can watch Wishes from an outdoor terrace that overlooks Magic Kingdom. It's an amazing view. If you don't want to go outside, a table by the window would be awesome because the restaurant dims the lights and plays the music so that Guests get the entire experience.
'Ohana does the same thing again, with a table by the window being optimal.
You can request a table by the window when you make your reservation, but it is never guaranteed. I would check in to whichever restaurant you choose at least 30 minutes earlier than your reservation time and tell them that you are willing to wait for a table by the window so you can see the fireworks. They will try their best to accommodate you.
I would choose 'Ohana because it's definitely more laid back, and a lot more fun for your young kids. Your six and four year old will love the coconut race. The food is all you care to eat, and it is DELICIOUS!
I hope this helps! Thanks for visiting the Moms Panel. Have a magical vacation!