I have been to St. Maarten several times and each time I go, I find something new and interesting. I think if you're shopping for gold/jewelry, you can do well in Phillipsburg.
As far as activities go, since you are traveling with kids I'd highly recommend the America's Cup Regatta too. It's a great experience and still one of our favorite excursions ever.
If you're not into that, you could contact Joyce Prince. She's a local tour operator that offers half day island tours and other services. She offers friendly service and is very knowledgeable. While we were on our tour last year, she took us to Maho Bay and we watched the planes take off at Princess Juliana Airport--what an experience. Anyway, if you want to email Joyce, she can at email@example.com