Arduino based toys have their place, but in the end, the shield-bot is a toy. Where the Pi shine's in my opinion is this: It packs the processing power of a small desktop and combines that with enough interface options (GPIO, Clock, AD and DA, I2C, etc, etc) to interface it with real world projects. True, some things might be a bit too complex for a kid, but the opportunity to play and learn together with your kids are endless!