National Geographic Kids has been great. It balances articles about animals, people and fun outdoor activities. I accidentally let this one lapse, and I'm still hearing about it.
Nickelodeon Junior is actually for parents of small children, or infants. My 6 year old was too mature for most of the items inside, and I didn't need another parenting magazine, so this one lapsed for good.
Kid's Discover is unusual in that it covers only one topic in great depth. We've received issues on Mexico, the underground railroad, samurai, the heart, etc. Some were interesting, some were a little dull, and my we found the heart and lung issues a little difficult to look at. Still, informative, entertaining, and absolutely wonderful for school papers and projects.