Nintendo has made clear their policy regarding its younger users and the Miiverse, in a recent update to the Miiverse code of conduct rules: "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."
So it's basically back to the way we used to do it on the Wii, you'll need to both directly add the Nintendo Network ID of the other person you wish to friend into your friends list, rather than making a friend request from Miiverse itself.
Nintendo also provided some guidelines for kids to follow when adding friends, recommending that they only add people that they know in real life, like friends from school or the neighbourhood. They also cautioned children not to attempt to share their Nintendo Network ID's over Miiverse.