Having worked in a cattery for many years i am going to disagree with others
We had many many indoors cats stay and the outside runs never caused a problem.
The thing you have to think about is, is it fair to keep your cats in the house compartment for 16days ?
Even a FAB registered Catterys house compartments arnt big enough for that (and they would be the largest you would find)
I also dont think it will pose a problem for when they come home as the whole situation is different