Lego City Undercover is quite possibly the very best Wii U game yet released. ETA: The amazing, creative story environment of Lego City has yet to be equaled on the Wii U IMHO.
Donkey Kong is good, but best for slightly older kids who will not get frustrated easily. My 10 year old loved it, but still got frustrated sometimes.
Pickmin 3 is just annoying. It has a timer you have to beat and upset my 8 year old so much (after he spent all of his birthday money on it) that I sold it for him on ebay and made up the difference in the money he lost because he was so devastated.