We arrived at the parks on a busy/summer Friday evening and I insisted to myself that it was just a "bonus" day, not a scheduled part-day in the parks. That way, I would not have been disappointed if we didn't get much done. Especially with flying, I wouldn't make any hard and fast plans about when you are going to get there and what you are going to do on Friday. Having some ideas (including fireworks if you were going to see them) is great, but a very detailed itinerary could lead to disappointment. So I prefer to think of that day as a bonus.
(By that reckoning, though, if I were in your shoes I would say that you're kind of only going to the
Disneyland Resort parks for 2 days--it's not too likely that you will be able to do "everything" in 2, or even 2.5, days. If your priority is doing Sea World,
Legoland, AND the Disneyland Resort, that's fine, but it will take away from Disneyland time and probably make things more rushed there.)
I don't have any children but for myself, I'd rather be in Disneyland than anywhere else!

But it might be worth it for your family to go to one of the other parks on Sunday or maybe Saturday--you know them better than I do.
I don't know your numbers but there's a good chance that it's also cheaper to go to DLR for an extra day than to drive down to San Diego to visit Sea World or Legoland.
I think Sunday (until late evening) may be busier in the Disneyland Resort than Saturday. Weekdays (Friday evening doesn't count) are generally less crowded than weekends--the park hours will likely be shorter but with the age of your kids, I'm thinking an 8 PM (or so) close time isn't an awful thing.
Oh, and Legoland appears to be closed the Tuesday of your trip, if you didn't know that.

There's an aquarium there, though--I don't know anything about it except that it exists, but I guess it could give you a little taste of water-life if that's what you were after. I'd do Legoland over Sea World just because Sea World seems common to me (there is certainly one in Orlando

).
Have a great time!