I think the Enchantment is the nicest of the ships doing 4 night cruises. Both March 9 and March 23 still have quad availability in inside, outside and balcony cabins, although not in every category. There's no way to know how long you can wait, especially if you're looking for a specific type of room or price point.
Having two rooms has the advantage of more space and especially an extra bathroom, not to mention some privacy for the adults. But it's more expensive. Your travel agent can tell you the difference in price so you can decide if it's worth it. I assume you would want either connecting or adjacent rooms, if they're still available. Some families get rooms across the hall from each other, such that the adults have an ocean view and the kids have an inside room. Again, if you want two cabins and have specific requirements like connecting rooms, the sooner you book, the better.