As to 1/2 day at the parks--I'd rather be at the ship sooner. You can board about 11:45 and go to lunch...then hit the pools. They are rather uncrowded then. You can't get to your cabin until 1:30, but you can explore your new home--check out the public areas, etc. I usually go down and unpack when the cabin is available. That way, everything is finished (including pressing anything that needed it), and after the safety drill I'll have no more work for the week.
It is an hour driving from WDW to port. My preference is to arrive at port before 11:30. I shoot for 10:30 if I need to make ressies, but 11:30 is fine if I've done everything in advance. That really doesn't allow for a park trip in the morning. However....I will do laundry the morning that we leave for the ship if I've spent the week before in the parks. That way everything is clean, etc....again, no worries while cruising.
No idea how many Easterns we've done. We have 15 sailings on the Magic, but I can't keep track of how many of what. We now schedule by dates (off school time) rather than where the ship is going.
We've done Coral World and the island tour--this is much easier and cheaper to do on your own. You can get discount admission coupons on line or in the tourist info book at the port. That way, you can stay at Coral World as long as you/kids want to. There is a lovely family beach right across the parking lot--if you wear swimsuits under your clothes, you can enjoy that too. It is very easy to get a cab back. Sorry, I can't quote current cab fare. Coral World has underwater viewing, aquarium, hiking trails, animal touching area, snack bar and gift shop. Good for ages 3-???.
Butterfly farm--again, do this on your own--cheaper and no crowds. The butterflies are much more active when the crowds leave. Again, good for all ages.
My favorite beach on St. Maarten is Little Bay. It is lovely, not as written up in tourist mags as some of the others, so much less crowded. With kids 10 and 12, avoid Orient Beach. There is supposedly a family friendly area, but there is definitely an area where "fat old men" walk around nude. Description is from my DD.
If you do shopping--or just walk around the shopping areas...don't miss a shop called "That Yoda Guy" or something similar....at least if your kids are into Star Wars. One of the original artists sells art work, movie misc., etc. He's friendly and it's a great fun place to just look.
There's great snorkeling and scuba available if the kids are adventurous..... I generally don't book excursions thru
DCL, but we've been there enough that I'm totally comfortable doing it on my own. It's easy, and never more than the DCL price. Everyone speaks English, the currency is dollars. You'll need cash for the taxi fare, all else can be plastic. Just be sure to be back at the ship on time.