I would forget any possibility of getting off St Thomas Friday night. I also feel like trying to catch the 7:45 is an early morning rush for you and the kids. Remember, you are on vacation. I suggest using a combination of what Manpot and GeorgeC1 suggested. Spend the night at Emerald Beach (just off the airport property). The hotel is decent. The food is decent and the kids can enjoy the beach. Then catch the 9am ferry from CA for the reason GeorgeC1 said above. I believe this option is the most stress free start to your vacation. I realize that may get you off the dock later than you had hoped. But stress free.

What about your return?