As others have said a console isn't defined by it's hardware or archictecture but the software provided for the console.
-A significant percentage of the Wii's best selling games have colourful , vibrant themes to them , this may not make the Wii "kiddy" but still contributes to the idea.
-The sports multiplatform titles such as FIFA are done in a kiddie theme as opposed to the more realistic looking PS3/360 versions , this is an example of how the industry views the Wii demographic "Fifa 09 All Play"
-The Wii has a bigger percentage of titles with kiddies themes or content as opposed to the PS3/360 which have much smaller percentages.
-That's not to say a hardcore gamer wouldn't have fun but this is more to do with the perception of the Wii than the actual reality.
-The PS3 has various factors going against it being considered a " kiddy's" consoles , it's price point , functionality , colour etc. Many of the biggest marketed 360 games are hardcore Gears of War , GTA IV , Halo 3 etc so this damages the perception that it might be a "kiddy's" console.