I suggest not going to St. Thomas at all. Go from Tortola to St. John and spend the night there. Have your kids take the St. John ferry from St. Thomas when they arrive. We did this in December and it was easy. To clear in at St. John you can either take a mooring in Maho Bay or another bay (if there is a North Swell you don't want to be in Cinnamon Bay as it gets rolly) and then take a cab to customs (make sure everyone on board goes to customs). Or you can take the boat to Cruz Bay and go right up to the dock at the customs office, clear and then find somewhere to overnight. Cruz Bay is really crowded without available mooring balls. You might also be able to clear in and out of Tortola when you arrive by ferry. We did it and the only issue was that the customs agent cleared us in, then made us wait for everyone from the ferry to clear and then she cleared us out. You have to have the boat paperwork and information to be able to clear out. Just ask the charter company for it.