The actual discount involved in "50% off for kids" is minimal. The previous Disney+ deal from 2 years ago included free fares for kids, and this is just half-price. It's significant for the families that use it, but not a big loss for
DCL, especially if families spend more on Bibbidi-Bobbidi-Boutique, souvenirs, excursions, and adult dining because they saved money on the room. There are also a lot of blackout dates, so it's not every cruise until June, or even most cruises until June.
From DCL's perspective, they have empty rooms that could be filled by 1 or 2 adults who have probably cruised with Disney before or by 2 adults and 2 kids who may be taking their first
Disney cruise. If the family with kids has a good time, they are likely to take more cruises over the next few years that they may not have thought about taking without the special deal for their first cruise. So giving the discount only for 2 adults sailing with kids taps into a new market, which may be a different group from the people who typically book *GT rates.
I am guessing the shorter cruises are more profitable for DCL, which is why there are so many shorter cruises. As more people take shorter cruises and are ready for longer cruises, DCL may increase the number of longer cruises.