Down payment is required at the time of booking.
But our finances are a little tight right now, so the agent making the bookings suggested that I place the booking for just one person (because the down payment is $250 per person I think). She said I could later add my wife and child and pay the additional $500 down payment later and I would keep my 10% discount and on-board credit. I think the only down side to this plan is that if 1st seating fills up before I add the other two, I could see where we might have to be bumped to 2nd seating if I wait too long.