Why not make an airline for people with kids and then they could deck the inside of the plane out to make it fun for kids complete with a flying tube sized playground, maybe a mini arcade, and definitely a ball pit. The pilots could even be those creepy ass animatronic creatures from Chuck-E-Cheese.
Or it's simply not economically viable to use half of the airplane as a playground, because parents will never be willing to spend 2x the amount on flight tickets because they're perpetually broke
They could also make every second row smaller child's seats and put them closer together to fit more people and children in the plane. Bonus: the parents can use the top of the kiddie seats as a footrest.