Dear Dayna:
We have been on the Disney Magic a few times and your four year old is a great age to really get into it. Do you know that you'll have to pay to have your two year old stay at the nursery if you so desire? (See my article on this site of Disney's magical nursery) We had my seven month old stay there are few times and were pleased with the staff and atmosphere.
As for excursions, here are some ideas:
St. Thomas: Take a cab to Magen's Bay (much cheaper than going on a shore excursions). It's a lovely, gentle beach that always rates as one of the finest in the Caribbean.
Your father in law may enjoy taking a short walk through town to the Paradise Point Tram for the ride up to the observation spot. There's a few shops here and a cafe but most importantly, there is a fabulous view of the island and especially your ship in the harbor!
Or, maybe you could split up and some of you take the two year old to the beach and some of the other adults take the four year old to either Coral World or the Atlantis Sub ride. I find that toddlers do best at the beach but maybe your older child would want to do an excursion with grandpa!
St. Maarten: St. Maarten is a little harder since some of the best beaches are nude or topless which may or may not be your cup of tea. Actually on our last visit to St. Maarten we just took the short water taxi from the ship into town and right when you get off is a nice beach. I was amazed at how clear and nice the water was.
I'd like to hear about your cruise upon your return. When do you sail?
Smooth sailing,
Luisa Frey Gaynor
Family Cruise Editor