"And just to note -- the overall price for the party of 5 is less expensive this way, and less expensive for the single aunt; however the fare for the child is actually more. Depending on who planned to pay for the cruise fares, they may need to make some concessions on that."
The first two people in a stateroom pay adult prices, no matter how old the second person. The child staying with the aunt will pay the adult price not the kid price. Staying with the rest of the fam, the kid would pay the kid price. While the overall cost of the 5 people would be less, it won’t be less for the fam of 4 if they pay the adult rate for the kid with auntie.