If they stay at home and not in a cattery, then iMO it would be best to keep them indoors for the duration with the catsitter/Mom popping in to feed and play. After all, if you take them to the cattery, they will in any case be indoors and in an even smaller confined space. My cats have access to a catproofed garden, but they are still kept indoors when we are not home or go on leave. Cats are notorious houdinis and I cannot be 101% certain that they wouldn't be able to override the catproofing if left to their own devices.

While I was away for a week last October, I left my outdoors-loving, beloved cat, Bruno in the capable hands of my husband and in-laws. The second day I was away, he was knocked down and killed on a road near our house. It was so absolutely terrible for everyone that I now don't let my cats out. I know that every time I go out, they will be waiting for me, safe and sound when I get home. Bruno was born in the wild and loved that way of life, even though he was a home-body come night time. Even knowing this, I wish every single day I'd kept him in.

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http://catscanjump.blogspot.com

Wendy, mum to:
Simba von Wurzbachtal, Turkish Van, born Austria, 5/9/10

RIP
Tarka, 1989 - 2008 my gentle Phileas Fogg
Bruno, 2008 - 2010 taken too soon my darling boy
Misi, 1993 - 25/3/2012 My brave, beautiful tabby