Hiking the Grand Canyon in march

As tinkerbell said, the weather will be a crap shoot, but mid March is a fine time to hike the canyon.  We've got snow and ice at the rim now, with plenty of mud.  It can be a very good time to do the canyon if you're prepared (be ready for temps from 75 to freezing all in the same day.)  You said you are planning a day hike to the bottom?  That's not a problem for someone in good shape, just carefully consider everyone in your family as the shortest route (Kaibab) is very steep and remember that the rim elevation is around 7,000 feet, as I recall, there isn't any water between the river and the rim either.  I wouldn't try to go all the way to the river and back with my family on a day hike....but I don't know your family.  You might also consider a hike to plateau point on the bright angel trail if you want something a little less challenging.  Whatever you decide have fun and start early.