To be fair just not that many people have Wii U, kids or adults. I don't know many parents rushing out to get the $300 machine for their kids (except me!).
My kid was very excited playing SM3DW, but he's pretty much stopped playing Wii U, even though we got some more games.
kid: "Can I play games?"
me: "Are you gonna play Wii U?"
kid: "No, can I play on your Nexus 7 or iPad?"
Mario Kart 8 may be the last hope!
Probably the Xbox 360 is a better kid console now. While it's also a very hardcore console, it's $179.00, has lots of cheap games for kids, and kids love Kinect.