We've even had 4 in the room, and whilst it did get rather cosy it was never an issue.
I would guess if your 10 yr old granddaughter is like any other 10 year old, then she is going to love sleeping in the upper bunk.
The one tip I would offer is to, and check with her first, move the ladder each night to the end of the bunk. The steward will make the room up with the ladder coming down between the other 2 beds, which can get in the way for those on the lower bunk.
But apart from that, go for it, and I'd put money her preferring the bunk, and that spare cash can give you some great toours.
Alan & Katrina
Last Cruise: P&O Oriana October 2015
Next: Queen Victoria January 2016
Golden, Grand(2), Caribbean, Royal, Sea(4+1 aborted)
Adonia(4+2), Arcadia, Artemis(2), Aurora(5), Azura(2), Britannia, Oceana(3), Oriana(12), Ventura(2), Victoria(5)
QM2(2), QE2(9), QV(6), QE, Caronia(2), Minerva, Minerva II, Fred.Olsen(8), NCL Jade, =X= Eclipse, HAL Ryndam, Aza.Journey