Aine, do you have cat crates? You might try keeping them in crates most of the time for a few days. Put all their favorite goodies, toys, rugs, etc. in the crate, and put the crate in a quiet place. You'll have to let them out to use the sand box,of course.

If possible, move the cats last, after most everything is set up, so that some things will at least smell correct. Don't bug them a lot for several days, just let them hide and maybe come out at night to do their own thing.