We've been a few times with our son when he was 3, 4, and 6. He really enjoyed Loterie Farm on our last visit in December. There's a kids area that has a little pool, some slides, and a small zipline. Kids have to pay to go, but adults are free. Aside from that, during past trips we've gone to Maho Beach, spent time along the boardwalk in Phillipsburg, and just played on the beaches near Orient. We've only stayed on the French side, so I'm sure there are more kid-friendly activities on the Dutch side I don't know about.

For sand toys, you may want to check with the place you're staying. We've mainly rented through Airbnb, and some apartments have had toys for kids and other beach supplies. We also brought our own car seat, and the bag we used for that was big enough for us to fit a few toys and floaties. I think most airlines allow you to check car seats for free, and no one has ever cared that we added a couple of extra things to the bag.