Have you also posted on the USVI forum? There is a pretty active STJ forum on Trip Advisor that might be helpful. I don't think you'll have much luck with fireworks on STJ though since it's mostly National Park. No fireworks are allowed on park lands. If they have fireworks on STT, you might see them from Christmas Cove on St James, aka the home of Pizza Pi, but check on those local forums for confirmation.

Back to the BVI, I've heard others mention using Scrub as a base for Trellis. You could get a slip at Scrub and then ferry over to Trellis, but then head back if the vibe isn't quite kid friendly. That would also solve the potential issue of winds and swells. The kids might enjoy the Scrub pool and resort for the day/afternoon too.

I'm pretty sure you can't lose no matter where you end up in the VI's. Good luck!