4 adults in a cabin is not necessarily my idea of fun! Cabins that hold 4 are not any bigger than cabins that hold 2. You'll have one bathroom and one TV, so you have no privacy and are always bumping into each other. Depending on the configuration of your cabin, you could end up with the two kids in trundle beds (single beds that pop down out of the ceiling and you have to climb a ladder to get into), a double fold-out couch, which you really wouldn't want to put two siblings into, or one trundle bed and a single sleeper sofa, neither of which are really all that comfortable.
Yes, putting 4 people in a cabin is cheaper than putting 2 people in two cabins, but you have to decide if sanity is more important than money.
You might think about two inside cabins, or perhaps one oceanview or balcony for you across from an inside cabin for the kids. It'll cost you more money, but is definitely more comfortable for everyone and will keep your sanity.
As I always tell people who want to do a cabin for 4 adults; if you're not really good friends when you go in there, you'll be really good enemies when you come out!
You really do have to consider what is more important to you; sailing as cheap as possible or sailing more comfortable?