OK, if you cancel their room by 45 days out, you'll forfeit the deposit amount for them. You may be able to rebook that room with your husband & one child, but they would pay the prevailing rate and each of them will be paying full fare.
If you move your husband & 1 child into their room, and have them be no shows, you'll forfeit their whole fare. And you and the other child will be paying full fare for your room.
Another caution, it's possible, if you opt to try moving your husband & child (keeping your parents on the booking), it's possible that room's lifeboat may be at capacity and you won't be able to move them at all. There's a slight possibility of that.
Also, you say "adjoining", mean "connecting"? As in an interior door between the two rooms? Adjoining means next door (or across the hall) from each other, but with no interior door.