Quoted Dimensions for the rear internal widths :
V50 - 1374mm
xc60 - 1403mm
So according to that you've got an extra 29mm than you do in the V50, to give you an idea of sizing.
Couldn't see figures for the Disco.
Similar discussion on here previously, although with the younger children.
https://www.volvoforums.org.uk/showthread.php?t=221607
Other thing to bear in mind, if anyone says it will or won't work, is that a lot will depend on the actual boosters you are using, as I think even between seats suitable for similar ages, there can be quite a difference in width depending on how much padding is put on them.
Only real way to tell is to try it. Go on Autotrader and find the closest xc60 for sale, and take the family.