The problem is that you will be missing them and worrying far more than they are, I suspect. Do you have someone else who they are used to who could come over occasionally for a cuddle with them? Failing that is there someone you know and trust who is a cat person but who may not be known to the cats yet who could occasionally go in? Cats can be very fickle and will probably accept a stranger who makes a fuss of them. I have noticed this over several years of looking after cats that the owners swear are nervous and unfriendly - as soon as the owner has left they're after cuddles! If you are really nervous about it the only real answer would be a cattery so at least you would know where they are.