The problem with your suggestion is that you may not be able to add the other family members later. The cruise fills, not only by number of beds in cabins, but also by numbers of children of each age group. Thus, you can always add an adult later, but you may not be able to add the children.
Of course, if you book with fewer people in each cabin but have the requisite number of cabins...and are then told that you can't add the children...you can always cancel the entire thing and collect your refund.