According to 21st century Catholic critics, what's so great about the Canticle of the Sun? A few years ago, I tried to explain to the 7th-8th graders in my catechism class how awesome St. Francis was.  I told 'em about him giving up all his stuff and rebuilding the Church etc... but, for whatever reason, when I had them read the canticle of the sun it didn't do anything for them. 

I reckon that in St. Francis' way, this is a very humble way of looking at creation.  But it more or less fails to resonate with me or my students.  What do 21st century Catholic critics (Pope Francis, for instance) say the Canticle of the Sun says to 21st century Christians?
You don't specify what English translation, interpretation, rendering –whatever word one uses– you read to the children, but you do link to the Wikipedia text, which is more a translation of the original language in which St Francis wrote, and I agree that it is not inspiring or beautiful poetry. But there are other attempts, one of which may be found here. There are certainly others.

But it could be simply that we do not live in a poetic age, and very few people appreciate –or even know– any poetry at all. Children weaned on video games and movies filled with series of explosions are not oriented to poetry nowadays.  As for trying to "explain how awesome St Francis was," you might want to show them the Franco Zeferelli film Brother Sun, Sister Moon which was shot on location in Italy. Children nowadays are accustomed to on-screen learning. And it is a beautiful film.