Megan, I'm betting it's a racoon. We put new sod down in our backyard and every morning we would walk out there and the sod would be rolled up. I think this happened every day for a week.
Can I mention that my theory was the correct one?? :-)
Megan, I'm betting it's a deer or deer (plural). Get some sand from the kids' sandbox and put it around the plants - wet it and you'll see prints. Voila!