June and July are good times and you might get a few free days.
That being said after sailing both locations extensively in my opinion you are doing the newbies a disfavor but not taking them to the BVI first. Go to the Grenadines when you are not responsible for the good time of others, have the time to lean the lay of the land and can enjoy the beauty for yourself. In the BVI everything is easy. Rarely true of everywhere else I have chartered.