So far a few of my personal favorites would be No More Heroes and Super Smash Brothers Brawl.
No More Heroes is definately not child appropriate as almost every other word in the game is some sort of swear.
Super Smash Brothers Brawl may be decent, but I'm not sure how long it would keep a 6 year old interested. It's a fighting game with a wide variety of nintendo characters and you can also play online. It does have different modes ranging from easy to hard, so it's possible he would get some enjoyment out of it but I'm not sure I'd recommend it.
I think you did good with just getting the Wii + Mario. I havn't played sonic but I'm sure it's a popular title as far as 6 year olds go. I would recommend getting at least 1 more Wii controller + possibly 2 nun-chucks to get you started. That way any 2-player games you come across you'll be able to play even if they require nun-chucks. A few other recommendations:
Wario Ware - Smooth Moves (Probably $50)
http://www.gamespot.com/wii/puzzle/wari ... ary;review
I don't know how difficult this would be for a 6 year old to pick up. Alot of the game is just randomly pushing A or shaking the controller the right way at the right time. This game is alot of fun once you get use to it and always a great party game so it has replay value.
Nyko Charge Station - $30ish
http://reviews.cnet.com/game-accessorie ... 63104.html
As you can see this can charge up to 2 controllers and considering I doubt your wii will be subject to large periods of use they should never run out of power between charges. This would save you the hassle of dealing with batteries.
Wii Play - $50ish?
http://en.wikipedia.org/wiki/Wii_Play
It's an extra controller and a game all wrapped up into one. The game doesn't have amazing replay value but it might add some amusement for now. It includes a variety of games with the notable ones being a shooting range (similar to duck hunt), tanks, and lazer hockey. I think the game loses it's replay value fairly quickly, but I think all of the games are 2-players. Also as I said it comes with an additional controller.