agreed. as long as credit is used properly there is no harm in opening accounts in children's names early. I'm glad my parents did. based on the weird way FICO works, my 10 y/o co-signed credit card account account allowed me to get my own card when I was in college without a cosigner and further helped to get better rates on loans for grad school.
if you plan on spending enough on your cards to get multiple RR credits, may as well spread it out and use the kids filghts to reach the level a few trips faster but keep in mind the FICO trick only works if the accounts remain open (which means annual fees for some cards)