Hello LittleMermaid04, welcome to the DIS boards!
I'll offer some advice, I've been to several Sea Worlds (including Orlando) and been to WDW 3 times in the past 6 years.
Twice we have taken an early morning flight to WDW and been able to check into our resort (Poly and BWI) around noon with no problems. If you are unable to check in they will store your luggage for you so you can go off to the parks.
With a park hopper pass I would only go into a park on arrival or departure day if you knew you could get your money's worth out of it because you will burn a full day off your park hopper. Both times we checked in at noon we had the old "length of stay" type passes now called Ultimate Park Hoppers (I think?). Last trip we had regular Park Hoppers and didn't want to burn a half day, actually less and we didn't get to our resort until 4pm. So we swam at the resort pool, we went out for a nice dinner at the BW and hung out there. My DD got her hair wrapped and DS and DH rented a surrey bike. We got to bed early and off to the parks the next morning.
Personally I wouldn't do a shortened day at Sea World because the trick there is getting the timing just right so you can see all the shows you want to. The Shamu show only runs a few times a day so you sort of plan around that show. If you didn't get there until noon you might have trouble with scheduling.
If you do go into a park for half a day with the age of your kids I would do MK. It is what the kids really want to see and do first. You could do some rides, see the afternoon parade (which would get that out of the way) and maybe have a nice dinner there.
That is my .02 ! It really is just a personal decision. Oh yea, some people like to do Downtown Disney on a half day. The
Lego Store is great, so is World of Disney, and the Toy store with the Mr. Potato Heads. You can eat at Rainforest Cafe too.
Good Luck.
