That looks cool but I wonder how it would work on a triple crank; would you set it as the outer ring, then put the 52 in the middle and I don't know a 39 on the inside? If you have to shift rings on the triple, why not just swap the 52 for a 48?

My 10 yr old's bike came w/ one of these:

http://store.somafab.com/ogkchainguards.html