This hopefully is a temporary rebellion. Bunnies don't like changes. Could you put clay litter in first---about an inch or two and then put Care Fresh on top of that? Hopefully they'd think it's all Care Fresh.... Or maybe they are rebelling against not being able to eat their litter anymore. Just remember who is the parent here!!!

Do they have paper they can rip up, like a phone directory or the sports section?

I'm betting this is a temporary clash of wills so don't sweat it. Easier said than done I know, esp. when they are in a snit and/or decide to "move" their litter box location!