Apparently, to Souplanation they think that a "family" is made up of 1 parent and 1 child. While I know that the divorce rate has been growing in recent years, I'm told that there still are families that actually have two parents, many of separate sexes, though in reading the paper that's not necessarily the case anymore. And many of those very same families have more than a single child. Maybe this offer is intended as a sort of participatory tag-team event, where parents and children trade off on any given night over who gets to go out to eat at Souplanation.
I do know that if there are two real parents they can each a coupon (if they're lucky enough to be approved as members). Though that would still limit them to two children. Perhaps Souplantation is just being socially conscious in promoting protection against overpopulation.
Then again, taking an even closer look still (continuing to be a good thing to do), and reading even further down, it turns out that children are defined as "12 and under." So, if you're a family of two parents three teenaged children, you are not actually a "family" per se in the eyes of Souplantation. Only one of the parents counts.
In which case, they have other coupons for you that are much better deals.