The 3DS is going to sell nowhere close to the DS, so that's the worrying trend if you're Nintendo is that more kids are probably growing up with these other kinds of portable devices.
Nintendo will always have a share of the kids market, but it's definitely moving into an eroding fashion, especially as tablets get even cheaper and more and more kids keep getting their first smartphone at a younger and younger age.
I didn't honestly get my first cell phone until I was 20 years old and the only game you could play on it was Snake, lol. If you had a "fancy phone" maybe you could play Tetris on a tiny screen.