From the budgeting side - 3rd and 4th guests in a cabin are less expensive than the first two guests. Your granddaughter’s fare would be slightly less.
This was for a Cabin on Deck 8. Price does go up as you go to higher decks.
Deck 6 was about $8 pp cheaper. So not a big jump.
View attachment 888556
If you were to opt for this category of cabin - but booked 2 of them, you would have the same rate for both cabins.
Total $7157.68.
There is no 3rd/4th person nor child discount when occupancy is 2 guests of any age.
View attachment 888557
Only your family can decide if more and separate sleeping space is worth $1500 for a 4-night vacation.
Would you ever consider staying together in a hotel room? That helps in deciding one or two cabins. These cabins are usually not as large as hotel rooms.
I find a first, short cruise like this means my room is a sleep and shower place. There’s a ship to discover.
Unless you choose the basic interior cabins, you will have a split bathroom. One unit has a tub and sink; the other is a toilet and sink. Each has a door. And you can go to the spa if two or more need the shower at the same time.
I like to pick a cabin surrounded by other cabins - above, below, beside and across. It usually means a quieter ship.