The FSW board was 57 cm, so the Starsufer S will be stable enough.
I don't feel your son will grow out of the Small *sufer quickly because he will get better too. The board will float him well for a few years. Main difference for him will be stability, but he will rely less and less on the board for stability as he gets better and stability will be less important.
One nice feature of the board is that there are holes in the bottom to attach a leach. You can hold your daughter with a long rope and she will feel secure, even with offshore wind.
The central fin is also very important. The kids are so light that the board rails don't grab at all. The central fin is the only thing that can keep them upwind. I've seen other kids planing on the same board with the fin attached, so it is not much of an hindrance.
Finally, the back fin supplied with the board is way too big for a small 2.0 or 2.5 sail. (The back and center are the same plastic fins) It was impossible for my kids to tack the board. The reason follows: If you look at the board sideways with the fins attached, you can estimate the position of the board Center of Lateral Resistance. You will see that if the kids bring back their small sail on the board, its Center of Effort will always stay in front of the CLR, preventing them from turning upwind. You need to get a smaller fin or to cut down one of the plastic ones. I found an old 20 cm wave fin that works good. With a small fin in the back, the CLR moves closer to the center fin. With the sail attached at the back of the track, the kids will now be able to tack or to jibe, at their choice!