My sister has owned a house on the dutch side of St. Maarten for about 18 years now.
We have been there a few times.
Our absolute favorite family beach is Orient Beach, it's located on the French side. This beach is actually on a bay so the water is calm, it has a great restaurant right on the beach called Kontiki, a little pricey but worth every penny. You can sit in glider bench tables and put your feet on the sand underneath, it's a wonderful feeling.
A beautiful beach is Cupecoy, it is on the ocean and has high rock cliffs, just get ready for the drop off in the ocean.
If you love food and want to sample St. Maarten's best then you must go to Grand Case, a beautiful French Street filled with some of the finest gourmet restaurants. One side all of the restaurants are on the water. Our favorite is The Fish Pot.
Here's a link to a good reference site for St. Maarten
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http://www.gobeach.com/page3.html
Hope it helps, we'll be doing the Western, guess i'll miss my sister, maybe next time