First, you must have administrator access and your kids should not. You must have control over their passwords, set them to expire, etc. They must know you have access to their accounts. They also will have separate folder space which you will be able to access, view files in cache, cookies, etc. I think if you can do this and show them how you can access their stuff, they might behave without much problem. Oh, also make sure they have no permission to delete even their own full permissions.