Clearly solution here is to increase price of tickets for underage children. Which is the fairest way for everyone.
Cost-wise, kids should be 50% off.
Make it more like a resort where you hand off the kids to the on-site daycare and enjoy a carefree day. Need a staffer, but the extra seating capacity should cover it.
We have on-board showers, individual cabins, sit down bars, but where’s the onsite daycare?