St. Maarten is an extremely easy beach island--take the water taxi from the same dock across great bay, and there is a beach at the little dock where you get off the water taxi--all w/i sight of the ship, so easy to go back & forth for lunch, for example, and the water taxi has a bracelet that is good for back & forth all day. and for mom, great shopping on front street is one block from the beach. Or an easy cab ride (share the cab, save costs) to orient beach which is more tropically gorgeous than the great bay beach, though the occasional au natural beach walker might not be gorgeous, but that didnt really impact our 9 yr old this year. the well organized cab stand is likewise adjacent to the dock and unlike some islands, the cabs are not 15 passenger trucks that wont leave until all full, so you dont have to sit around.
St. Thomas has gorgeous beaches, though I really would skip the st. john side trek as it is a schlep for the younger ones, lovely destination or not. we did magan's bay on our own, lovely sand and gorgeous, calm water, easy to do on your own. many will say the shopping on st. thomas is better than st. maarten, though I prefer st. maarten, but either is good if you are interested. there is an extra day at sea compared to the western, which gives the girls more chance to enjoy the on-board, in the clubs programming, and more chance for mome to enjoy the spa. the extra day at sea is typically calm, as the rough sea days are when you cross the gulfstream, so it is not an extra day to worry about seasickness as some worry.
there is an issue w/ early wakeup on st. thomas for immigration. so, ...just get up early and get a jump on your day--nap later if you need to, thats the benefit of the oversized
DCL staterooms.