We've sailed twice in January (Magic 02 and 04) and are sailing the Magic again in two weeks. Our experience is that the ship is always near capacity, and there are always a lot of children, but it seems like there are slightly more younger, preschool children in January, which makes some sense because school is in session at that time. We've always had a great time, and the only time we felt that it was crowded was at the pools on sea days.
Have a great trip!