San Juan: I could skip this port and NOT miss anything. Like going to an decaying American city in the middle of the Caribbean. We walked to El Murro. Good exercise but did not feel particularly safe. Nice to see the police standing on many of the intersections. The shops along the way reminded me of going to Dollar General or the Dollar store. Cheap merchandise....mostly trinkets. I would recommend booking an excursion if you want something to do in San Juan. I will not be going back there anytime soon.
St Maartens: We had booked an excursion through the cruise ship, SMH6 Catamaran Mirabella Beach and Sea excursion. Sailing was great, crew was great, the beach was beautiful, and the snorkeling was fun but not the best in the Caribbean. Water was rough and not much to see. Did not bother me because the entire experience was fun and delightful.
Shopping in St Maartens at the shops just off the pier was very good. Many shops, safe area and very clean as opposed to San Juan.
St. Kitts: Shopping was very good at the shops just a short walk from the pier. We also booked the bus/train ride around the island: Caribbean Scenic Railway Tour (KT15A). Very informative tour of the entire island. The train ride was very pleasant with drinks offered and entertainment(singers) provided. A great way to experience and see St. Kitts. Not sure how young children would enjoy this excursion but it was great for us mature adults.