Is this a good idea?

Sound good! Really creative. But make sure to check on them everyday to make sure the sand isn't giving them infections (getting in their eyes, nose, ears etc.) and make sure it isn't dirtying up their fur. They should have a fun time in it. But keep in mind that the little parts may not be as easy to clean as a bar cage, and you must keep them regularly clean. Hamsters usually wont use the restroom in tubes. And if you're getting multiplke hamsters, be cautious.