They'll hurt you if one rolls over you. If you've had a couple of beers, your reaction time isn't what it is normally. You may think you have enough time to grab that doubloon before the float gets here, but it's closer than you think. Hardly a year goes by where there isn't one person hospitalized because they were hit by or rolled over by a float. Kids love parades.
Locals bring out ladders and attach seats to the top so the kids can sit up and get a good view.
Even if you don't have a ladder, put your kid on your shoulders and they'll be able to see the floats and catch the stuff. Buy 'em a bag of caramel corn from a vendor before the parade gets there, walk up and down the parade route a block or two and let them see all of the people. We're all acting like kids anyway, so the kids will fit right in. There are over half a million people on Canal Street alone on Mardi Gras. Don't ever take your eyes off of your child.