This may be a bit controversial, but I'd be hesitant to offer her the kids 1-6pm on Christmas Eve. If she decided to break the deal and keep them, it'd wreck his holiday. I don't like to put myself into vulnerable positions when dealing with aggressive people. I'd be more willing to offer her earlier access e.g., she gets them from 10am Xmas and returns them at 2pm the next day.
PS - Kids are usually not opposed to waking early than usual on Christmas.
