You can book non-gamblers through URComped, including children. However, cruise lines will typically not comp minors unless there is a special promotion which allows it. If you’re a single adult traveling with a child, your fare can be comped but your child’s fare will not be. If you’re traveling with two adults and one child, the two adults can be comped but the minor’s cruise will be full price.
When you add extra guests to a comped cabin (double occupancy), the price might go up significantly because larger cabins, also called “quad” cabins, are limited in number and usually cost more.