Nintendo has been very forthcoming since the last E3 that they want to keep the Miiverse a friendly atmosphere, which includes restricting posts users make if they contain spoilers or are inflammatory toward other users. In a recent update to the Miiverse, Nintendo has imposed yet another rule to keep things friendly—users 12 and under can no longer send friend requests through the Miiverse.
The Miiverse Code of Conduct now reads, "For the protection of younger users, direct friend requests are not possible in Miiverse for users aged 12 and under. At the same time, younger users can make friends on Wii U outside Miiverse by entering each other's Network IDs in the friend list on the HOME Menu."
In other words, young users can still add people to their friends lists via the Home menu, but they cannot fully interact with these friends through the Miiverse.
Nintendo further stated in the Code of Conduct that this new rule is to help ensure that Miiverse friends for young kids are friends they have in real life, and not strangers on the Internet. When you're 13, feel free to meet strangers online, but when you're young, we'll do our best to shelter you from stranger danger.
The kids may not like this new rule, but they're under 13; they have plenty of time to make mistakes online later. Parents should be very happy with this change in the code in conduct, especially since it means they won't have to monitor their online interactions as much as they should have in the past. (Btw parents, you should still monitor your kids' online interactions.)